Semiconductor device and display device including the semiconductor device

ABSTRACT

A novel semiconductor device including an oxide semiconductor is provided. In particular, a planar semiconductor device including an oxide semiconductor is provided. A semiconductor device including an oxide semiconductor and having large on-state current is provided. The semiconductor device includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The first region and the second region have different impurity element concentrations. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 14/582,273, filed Dec. 24, 2014, now allowed, which claims the benefit of foreign priority applications filed in Japan as Serial No. 2013-271312 on Dec. 27, 2013, and Serial No. 2014-037207 on Feb. 27, 2014, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

One embodiment of the present invention relates to a semiconductor device including an oxide semiconductor film and a display device including the semiconductor device.

Note that one embodiment of the present invention is not limited to the above technical field. The technical field of one embodiment of the invention disclosed in this specification and the like relates to an object, a method, or a manufacturing method. In addition, the present invention relates to a process, a machine, manufacture, or a composition of matter. In particular, the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a storage device, a driving method thereof, or a manufacturing method thereof.

Note that in this specification and the like, a semiconductor device refers to any device that can function by utilizing semiconductor characteristics. A semiconductor element such as a transistor, a semiconductor circuit, an arithmetic device, and a memory device are each one embodiment of a semiconductor device. An imaging device, a display device, a liquid crystal display device, a light-emitting device, an electro-optical device, a power generation device (including a thin film solar cell, an organic thin film solar cell, and the like), and an electronic appliance may each include a semiconductor device.

2. Description of the Related Art

Attention has been focused on a technique for forming a transistor using a semiconductor thin film formed over a substrate having an insulating surface (also referred to as thin film transistor (TFT)). The transistor is used in a wide range of electronic devices such as an integrated circuit (IC) or an image display device (display device). A semiconductor material typified by silicon is widely known as a material for a semiconductor thin film that can be used for a transistor. As another material, an oxide semiconductor has been attracting attention.

For example, Patent Document 1 discloses a technique in which a transistor is manufactured using an amorphous oxide containing In, Zn, Ga, Sn, and the like as an oxide semiconductor.

REFERENCE

Patent Document 1: Japanese Published Patent Application No. 2006-165529

SUMMARY OF THE INVENTION

As a transistor including an oxide semiconductor film, an inverted staggered transistor (also referred to as a transistor having a bottom-gate structure), a planar transistor (also referred to as a transistor having a top-gate structure), and the like are given. In the case where a transistor including an oxide semiconductor film is used for a display device, an inverted staggered transistor is used more often than a planar transistor because a manufacturing process thereof is relatively simple and manufacturing cost thereof can be kept low. However, signal delay or the like is increased by parasitic capacitance that exists between a gate electrode and source and drain electrodes of an inverted staggered transistor and accordingly image quality of a display device degrades, which has posed a problem, as an increase in screen size of a display device proceeds, or a display device is provided with a higher resolution image (for example, a high-resolution display device typified by 4 k×2 k pixels (3840 pixels in the horizontal direction and 2048 pixels in the perpendicular direction) or 8 k×4 k pixels (7680 pixels in the horizontal direction and 4320 pixels in the perpendicular direction)). Furthermore, as another problem, the occupation area of an inverted staggered transistor is larger than that of a planar transistor. Thus, with regard to a planar transistor including an oxide semiconductor film, development of a transistor which has a structure with stable semiconductor characteristics and high reliability and which is formed by a simple manufacturing process is desired.

In view of the above problems, one embodiment of the present invention is to provide a novel semiconductor device including an oxide semiconductor, particularly to provide a planar type semiconductor device including an oxide semiconductor. Another object is to provide a semiconductor device including an oxide semiconductor in which on-state current is high, to provide a semiconductor device including an oxide semiconductor in which off-state current is low, to provide a semiconductor device including an oxide semiconductor that occupies a small area, to provide a semiconductor device including an oxide semiconductor that has a stable electrical characteristic, to provide a semiconductor device including an oxide semiconductor that has high reliability, to provide a novel semiconductor device, or to provide a novel display device.

Note that the description of the above object does not disturb the existence of other objects. In one embodiment of the present invention, there is no need to achieve all the objects. Objects other than the above objects will be apparent from and can be derived from the description of the specification and the like.

One embodiment of the present invention is a semiconductor device that includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode that are in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film therebetween. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The first region and the second region have different impurity element concentrations. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

Another embodiment of the present invention is a semiconductor device that includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode that are in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film therebetween. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The first region and the second region have different impurity element concentrations. The second region is in contact with a nitride insulating film. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

In each of the above-described structures, the impurity element concentration of the second region is preferably higher than the impurity element concentration of the first region. In addition, in each of the above-described structures, the impurity element is preferably any one of hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorine.

In the above-described structure, the nitride insulating film is preferably a silicon nitride film.

Another embodiment of the present invention is a semiconductor device that includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode that are in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film therebetween. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The hydrogen concentration of the second region is higher the hydrogen concentration of the first region. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

Another embodiment of the present invention is a semiconductor device that includes an oxide insulating film, an oxide semiconductor film over the oxide insulating film, a source electrode and a drain electrode that are in contact with the oxide semiconductor film, a gate insulating film between the source electrode and the drain electrode, and a gate electrode overlapping the oxide semiconductor film with the gate insulating film therebetween. The oxide semiconductor film includes a first region overlapped with the gate electrode and a second region not overlapped with the gate electrode, the source electrode, and the drain electrode. The first region has higher crystallinity than the second region. The gate electrode, the source electrode, and the drain electrode contain the same metal element.

In each of the above-described structures, at least a part of the gate electrode and parts of the source and drain electrodes are preferably formed on the same plane.

In each of the above-described structures, the oxide semiconductor film preferably includes an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). It is preferable that, in each of the above-described structures, the oxide semiconductor film include a crystal part and that a c-axis of the crystal part be aligned parallel to a normal vector of a surface where the oxide semiconductor film is formed.

Another embodiment of the present invention is a display device including the semiconductor device having any of the above-described structures.

One embodiment of the present invention can provide a novel semiconductor device including an oxide semiconductor. In particular, a planar type semiconductor device including an oxide semiconductor can be provided. A semiconductor device including an oxide semiconductor in which on-state current is high can be provided, a semiconductor device including an oxide semiconductor in which off-state current is low can be provided, a semiconductor device including an oxide semiconductor that occupies a small area can be provided, a semiconductor device including an oxide semiconductor that has a stable electrical characteristic can be provided, a semiconductor device including an oxide semiconductor that has high reliability can be provided, a novel semiconductor device can be provided, or a novel display device can be provided.

Note that the description of these effects does not disturb the existence of other effects. One embodiment of the present invention does not necessarily achieve all the effects listed above. Other effects will be apparent from and can be derived from the description of the specification, the drawings, the claims, and the like.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIG. 2 is a cross-sectional view illustrating one embodiment of a semiconductor device.

FIGS. 3A to 3C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 4A to 4C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 5A to 5C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 6A to 6D are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 7A to 7D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 8A to 8D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 9A and 9B are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 10A to 10C are cross-sectional views each illustrating one embodiment of a semiconductor device.

FIGS. 11A and 11B each illustrate a band structure.

FIGS. 12A to 12C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIG. 13 is a cross-sectional view illustrating one embodiment of a semiconductor device.

FIGS. 14A to 14C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 15A to 15C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 16A to 16C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 17A to 17C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 18A and 18B are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 19A to 19D are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 20A and 20B are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 21A to 21C are cross-sectional views each illustrating one embodiment of a semiconductor device.

FIGS. 22A to 22C are cross-sectional views illustrating an example of a manufacturing process of a semiconductor device.

FIGS. 23A and 23B are a top view and a cross-sectional view illustrating one embodiment of a semiconductor device.

FIGS. 24A and 24B are a top view and a cross-sectional view illustrating one embodiment of a semiconductor device.

FIGS. 25A to 25C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 26A to 26C are a top view and cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 27A and 27B are top views each illustrating one embodiment of a display device.

FIG. 28 is a cross-sectional view illustrating one embodiment of a display device.

FIG. 29 is a cross-sectional view illustrating one embodiment of a display device.

FIG. 30 is a cross-sectional view illustrating one embodiment of a display device.

FIGS. 31A to 31C are a block diagram and circuit diagrams illustrating a display device.

FIG. 32 illustrates a display module.

FIGS. 33A to 33H illustrate electronic appliances.

FIGS. 34A to 34C are Cs-corrected high-resolution TEM images of a cross section of a CAAC-OS and FIG. 34D is a schematic cross-sectional view of the CAAC-OS.

FIGS. 35A to 35D are Cs-corrected high-resolution TEM images of a plane of a CAAC-OS.

FIGS. 36A to 36C show structural analyses of a CAAC-OS and a single crystal oxide semiconductor by XRD.

FIGS. 37A and 37B are a top view and a cross-sectional view illustrating an example of a connection portion.

FIGS. 38A and 38B are cross-sectional views each illustrating one embodiment of a semiconductor device.

FIGS. 39A and 39B are cross-sectional views illustrating one embodiment of a semiconductor device.

FIGS. 40A and 40B are a top view and a cross-sectional view illustrating an example of a connection portion.

FIG. 41 shows temperature dependence of resistivity.

FIGS. 42A and 42B show electron diffraction patterns of a CAAC-OS.

FIG. 43 shows a change in crystal part of an In—Ga—Zn oxide induced by electron irradiation.

FIGS. 44A and 44B are schematic views illustrating deposition models of a CAAC-OS and an nc-OS.

FIGS. 45A to 45C illustrate an InGaZnO₄ crystal and a pellet.

FIGS. 46A to 46D are schematic views illustrating deposition models of a CAAC-OS.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments will be described with reference to the accompanying drawings. Note that the embodiments can be implemented with various modes. It will be readily appreciated by those skilled in the art that modes and details can be changed in various ways without departing from the spirit and scope of the present invention. Therefore, the present invention therefore should not be construed as being limited to the following description of the embodiments.

In the drawings, the size, the layer thickness, or the region is exaggerated for clarity in some cases. Therefore, the scale of each component is not necessarily limited to that in the drawing. Note that the drawings are schematic views showing ideal examples, and embodiments of the present invention are not limited to the shapes or the values in the drawings.

In this specification, ordinal numbers such as “first”, “second”, and “third” are used in order to avoid confusion among components, and thus do not limit the number of the components.

In this specification, terms for describing arrangement, such as “over” “above”, “under”, and “below”, are used for convenience in describing a positional relation between components with reference to drawings. The positional relation between components is changed as appropriate in accordance with a direction in which each component is described. Thus, the positional relation is not limited to that described with a term used in this specification and can be explained with another term as appropriate depending on the situation.

In this specification and the like, a transistor is an element having at least three terminals of a gate, a drain, and a source. In addition, the transistor has a channel region between a drain (a drain terminal, a drain region, and a drain electrode) and a source (a source terminal, a source region, and a source electrode), and current can flow through the drain region, the channel region, and the source region. Note that in this specification and the like, a channel region refers to a region through which current mainly flows.

Furthermore, functions of a source and a drain might be switched when transistors having different polarities are employed or a direction of current flow is changed in circuit operation, for example. Therefore, the terms “source” and “drain” can be switched in this specification and the like.

Note that in this specification and the like, the expression “electrically connected” includes the case where components are connected through an “object having any electric function”. There is no particular limitation on an “object having any electric function” as long as electric signals can be transmitted and received between components that are connected through the object. Examples of an “object having any electric function” are a switching element such as a transistor, a resistor, an inductor, a capacitor, and elements with a variety of functions as well as an electrode and a wiring.

Embodiment 1

In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device are described with reference to FIGS. 1A to 1C, FIG. 2, FIGS. 3A to 3C, FIGS. 4A to 4C, FIGS. 5A to 5C, FIGS. 6A to 6D, FIGS. 7A to 7D, FIGS. 8A to 8D, FIGS. 9A and 9B, FIGS. 10A to 10C, and FIGS. 11A and 11B.

<Structure 1 of Semiconductor Device>

In FIGS. 1A to 1C, a transistor having a top-gate structure is illustrated as an example of a transistor included in a semiconductor device.

FIGS. 1A to 1C are a top view and cross-sectional views of a transistor 150 included in a semiconductor device. FIG. 1A is a top view of the transistor 150, FIG. 1B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 1A, and FIG. 1C is a cross-sectional view taken along the dashed dotted line X1-X2 in FIG. 1A. Note that in FIG. 1A, a substrate 102, an insulating film 104, an insulating film 108, an insulating film 116, an insulating film 118, and the like are omitted for simplicity. In other top views in the following description, some components might be omitted as in the case of the transistor 150. The direction of the dashed dotted line X1-X2 may be called a channel length direction, and the direction of the dashed dotted line Y1-Y2 may be called a channel width direction.

The transistor 150 illustrated in FIGS. 1A to 1C includes an oxide semiconductor film 106 over the insulating film 104 formed over the substrate 102, the insulating film 108 in contact with the oxide semiconductor film 106, a conductive film 110 in contact with the oxide semiconductor film 106 in part of an opening portion 140 a in the insulating film 108, a conductive film 112 in contact with the oxide semiconductor film 106 in part of an opening portion 140 b in the insulating film 108, and a conductive film 114 overlapping the oxide semiconductor film 106 with the insulating film 108 therebetween. Note that in the structure illustrated in FIGS. 1A to 1C, the insulating films 116 and 118 are provided over the transistor 150.

In the oxide semiconductor film 106, an element that forms an oxygen vacancy is contained in a region that is not overlapped with the conductive film 110, the conductive film 112, and the conductive film 114. Hereinafter, the elements which form oxygen vacancies are described as impurity elements. Typical examples of impurity elements are hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, chlorine, and rare gas elements. Typical examples of rare gas elements are helium, neon, argon, krypton, and xenon.

When the impurity element is added to the oxide semiconductor film, a bond between a metal element and oxygen in the oxide semiconductor film is cut, whereby an oxygen vacancy is formed. Alternatively, when the impurity element is added to the oxide semiconductor film, oxygen bonded to a metal element in the oxide semiconductor film is bonded to the impurity element, and the oxygen is released from the metal element, whereby an oxygen vacancy is formed. As a result, carrier density is increased in the oxide semiconductor film, and the oxide semiconductor film has higher conductivity.

When hydrogen is added to an oxide semiconductor in which an oxygen vacancy is formed by addition of an impurity element, hydrogen enters an oxygen vacant site and forms a donor level in the vicinity of the conduction band. As a result, the conductivity of the oxide semiconductor is increased, so that the oxide semiconductor becomes a conductor. An oxide semiconductor having become a conductor can be referred to as an oxide conductor. Oxide semiconductors generally have a visible light transmitting property because of their large energy gap. An oxide conductor is an oxide semiconductor having a donor level in the vicinity of the conduction band. Therefore, the influence of absorption due to the donor level is small, and an oxide conductor has a visible light transmitting property comparable to that of an oxide semiconductor.

The temperature dependence of the resistivity of a film formed using an oxide conductor (hereinafter, oxide conductor film) is described with reference to FIG. 41.

Samples each including an oxide conductor film are fabricated. As the oxide conductor film, the following oxide conductor layers are formed: an oxide conductor film (OC_SiN_(x)) formed in such a manner that an oxide semiconductor film is made in contact with a silicon nitride film; an oxide conductor film (OC_Ar dope+SiN_(x)) formed in such a manner that argon is added to an oxide semiconductor film with a doping apparatus and the oxide semiconductor film is made in contact with a silicon nitride film; and an oxide conductor film (OC_Ar plasma+SiN_(x)) formed in such a manner that an oxide semiconductor film is exposed to argon plasma in a plasma treatment apparatus and the oxide semiconductor film is made in contact with a silicon nitride film. Note that the silicon nitride films contain hydrogen.

A method for fabricating the sample including the oxide conductor film (OC_SiN_(x)) is described below. A 400-nm-thick silicon oxynitride film is formed over a glass substrate by a plasma CVD method and then exposed to oxygen plasma to add oxygen ions to the silicon oxynitride film, so that the silicon oxynitride film can release oxygen by being heated. Then, a 100-nm-thick In—Ga—Zn oxide film is formed over the silicon oxynitride film by a sputtering method using a sputtering target with an atomic ratio of In to Ga and Zn of 1:1:1.2, subjected to heat treatment at 450° C. in a nitrogen atmosphere, and then subjected to heat treatment at 450° C. in a mixed gas atmosphere of nitrogen and oxygen. After that, a 100 nm thick silicon nitride film is formed by a plasma CVD method. Then, heat treatment is performed at 350° C. in a mixed gas atmosphere of nitrogen and oxygen.

A method for fabricating the sample including the oxide conductor film (OC_Ar dope+SiN_(x)) is described below. A 400-nm-thick silicon oxynitride film is formed over a glass substrate by a plasma CVD method and then exposed to oxygen plasma to add oxygen ions to the silicon oxynitride film, so that the silicon oxynitride film can release oxygen by being heated. Then, a 100-nm-thick In—Ga—Zn oxide film is formed over the silicon oxynitride film by a sputtering method using a sputtering target with an atomic ratio of In to Ga and Zn of 1:1:1.2, subjected to heat treatment at 450° C. in a nitrogen atmosphere, and then subjected to heat treatment at 450° C. in a mixed gas atmosphere of nitrogen and oxygen. Then, argon is added to the In—Ga—Zn oxide film at an acceleration voltage of 10 kV and a dose of 5×10¹⁴/cm² with a doping apparatus, whereby oxygen vacancies are formed in the In—Ga—Zn oxide film. After that, a 100 nm thick silicon nitride film is formed by a plasma CVD method. Then, heat treatment is performed at 350° C. in a mixed gas atmosphere of nitrogen and oxygen

A method for fabricating the sample including the oxide conductor film (OC_Ar plasma+SiN_(x)) is described below. A 400-nm-thick silicon oxynitride film is formed over a glass substrate by a plasma CVD method and then exposed to oxygen plasma, so that the silicon oxynitride film can release oxygen by being heated. Then, a 100-nm-thick In—Ga—Zn oxide film is formed over the silicon oxynitride film by a sputtering method using a sputtering target with an atomic ratio of In to Ga and Zn of 1:1:1.2, subjected to heat treatment at 450° C. in a nitrogen atmosphere, and then subjected to heat treatment at 450° C. in a mixed gas atmosphere of nitrogen and oxygen. Then, in a plasma treatment apparatus, argon plasma is generated and accelerated argon ions are made to collide with the In—Ga—Zn oxide film, whereby oxygen vacancies are formed. After that, a 100 nm thick silicon nitride film is formed by a plasma CVD method. Then, heat treatment is performed at 350° C. in a mixed gas atmosphere of nitrogen and oxygen.

FIG. 41 shows measured resistivity of each sample. Here, the resistivity is measured by the four-probe Van der Pauw method. In FIG. 41, the horizontal axis represents measurement temperature, and the vertical axis represents resistivity. Square represent the measurement results of the oxide conductor film (OC_SiN_(x)), circles represent the measurement results of the oxide conductor film (OC_Ar dope+SiN_(x)), and triangles represent the measurement result of the oxide conductor film (OC_Ar plasma+SiN_(x)).

Note that although not shown in the graph, an oxide semiconductor film that is not in contact with a silicon nitride film has high resistivity that is difficult to measure. This indicates that the oxide conductor film has lower resistivity than the oxide semiconductor film.

As shown in FIG. 41, the oxide conductor film (OC_Ar dope+SiN_(x)) and the oxide conductor film (OC_Ar plasma+SiN_(x)) have small variations in resistivity because the oxide conductor films contain oxygen vacancies and hydrogen. Typically, the range of the variations in resistivity of the oxide conductor films at temperatures from 80 K to 290 K is from more than −20% to less than +20%. Alternatively, the range of the variations in resistivity of the oxide conductor films at temperatures from 150 K to 250 K is from more than −10% to less than +10%. In other words, the oxide conductor is a degenerate semiconductor and it is suggested that the conduction band edge agrees with or substantially agrees with the Feimi level. Thus, when the oxide conductor film is used for a source region and a drain region of a transistor, an ohmic contact is made between the oxide conductor film and conductive films functioning as a source electrode and a drain electrode, so that the contact resistance between the oxide conductor film and the conductive films functioning as a source electrode and a drain electrode can be reduced. In addition, since the temperature dependence of the resistivity of an oxide conductor is low, the amount of change in the contact resistance between the oxide conductor film and the conductive films functioning as a source electrode and a drain electrode is small; thus, a highly reliable transistor can be manufactured.

FIG. 2 is an enlarged view of the oxide semiconductor film 106 and the vicinity thereof. As illustrated in FIG. 2, the oxide semiconductor film 106 includes regions 106 a in contact with the conductive film 110 and the conductive film 112, regions 106 b in contact with the insulating film 116, and regions 106 c and a region 106 d that are overlapped with the insulating film 108.

The regions 106 a serve as a source region and a drain region. In the case where the conductive film 110 and the conductive film 112 are formed using a conductive material which is easily bonded to oxygen, such as tungsten, titanium, aluminum, copper, molybdenum, chromium, tantalum, an alloy of any of these, or the like, oxygen contained in the oxide semiconductor film is bonded to the conductive material contained in the conductive film 110 and the conductive film 112, and an oxygen vacancy is formed in the oxide semiconductor film. Furthermore, in some cases, constituent elements of the conductive material that forms the conductive film 110 and the conductive film 112 are partly mixed into the oxide semiconductor film. As a result, the regions 106 a in contact with the conductive film 110 and the conductive film 112 have higher conductivity and serve as a source region and a drain region.

The region 106 b and the region 106 c serve as low-resistance regions. Impurity elements are contained in the region 106 b and the region 106 c. Note that the impurity element concentration of the region 106 b is higher than the impurity element concentration of the region 106 c. In the case where a side surface of the conductive film 114 has a tapered shape, part of the region 106 c may be overlapped with the conductive film 114.

In the case where the impurity element is a rare gas element and the oxide semiconductor film 106 is formed by a sputtering method, the regions 106 a to 106 d each contain a rare gas element. In addition, the rare gas element concentration of each of the regions 106 b and 106 c is higher than that of each of the regions 106 a and 106 d. The reasons are as follows: in the case where the oxide semiconductor film 106 is formed by a sputtering method, a rare gas is used as a sputtering gas, so that the oxide semiconductor film 106 contains the rare gas; and a rare gas is intentionally added to the regions 106 b and 106 c in order to form oxygen vacancies in the regions 106 b and 106 c. Note that a rare gas element different from that added to the regions 106 a and 106 d may be added to the regions 106 b and 106 c.

In the case where the impurity element is boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, or chloride, the impurity element is contained in only the regions 106 b and 106 c. Thus, the impurity element concentration of each of the regions 106 b and 106 c is higher than the impurity element concentration of each of the regions 106 a and 106 d. Note that, in the region 106 b and the region 106 c, the impurity element concentration, which is measured by secondary ion mass spectrometry (SIMS), can be higher than or equal to 5×10¹⁸ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³, higher than or equal to 1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²¹ atoms/cm³, or higher than or equal to 5×10¹⁹ atoms/cm³ and lower than or equal to 5×10²⁰ atoms/cm³.

In the case where the impurity element is hydrogen, the impurity element concentration of each of the regions 106 b and 106 c is higher than the impurity element concentration of each of the regions 106 a and 106 d. Note that, in the region 106 b and the region 106 c, the concentration of hydrogen, which is measured by SIMS, can be higher than or equal to 8×10¹⁹ atoms/cm³, higher than or equal to 1×10²⁰ atoms/cm³, or higher than or equal to 5×10²⁰ atoms/cm³.

Since the region 106 b and the region 106 c contain the impurity elements, the number of oxygen vacancies is increased and the carrier density is increased. As a result, the region 106 b and the region 106 c have higher conductivity and serve as low-resistance regions.

Note that the impurity elements may be one or more of hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorine, and one or more of rare gases. In this case, the conductivity of the region 106 b and the region 106 c is further increased in some cases by interaction between an oxygen vacancy formed by the rare gas and one or more of hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorine that is added to the regions.

The region 106 d serves as a channel.

In the insulating film 108, a region that overlaps the oxide semiconductor film 106 and is overlapped with the conductive film 114 serves as a gate insulating film. In addition, in the insulating film 108, regions that overlap the oxide semiconductor film 106 and are overlapped with the conductive films 110 and 112 serve as interlayer insulating films.

The conductive film 110 and the conductive film 112 serve as a source electrode and a drain electrode. The conductive film 114 serves as a gate electrode.

In the transistor 150 described in this embodiment, the regions 106 b and 106 c serving as low-resistance regions are provided between the region 106 d serving as a channel and the region 106 a serving as a source region or a drain region. The resistance between the channel and each of the source region and the drain region can be reduced, and the transistor 150 has high on-state current and high field-effect mobility.

In a process of manufacturing the transistor 150, the conductive film 114 that serves as a gate electrode and the conductive films 110 and 112 that serve as a source electrode and a drain electrode are formed through the same step. For that reason, in the transistor 150, the conductive film 114 does not overlap the conductive films 110 and 112, which can reduce the parasitic capacitance between the conductive film 114 and each of the conductive films 110 and 112. As a result, in the case where a large substrate is used as the substrate 102, signal delay in the conductive films 110, 112, and 114 can be reduced.

In the transistor 150, the impurity element is added to the oxide semiconductor film 106 using the conductive films 110, 112, and 114 as masks. That is, the low-resistance regions can be formed in a self-aligned manner.

Details of other components illustrated in FIGS. 1A to 1C are described below.

As the substrate 102, any of a variety of substrates can be used without particular limitation. Examples of the substrate include a semiconductor substrate (e.g., a single crystal substrate or a silicon substrate), an SOI substrate, a glass substrate, a quartz substrate, a plastic substrate, a metal substrate, a stainless steel substrate, a substrate including stainless steel foil, a tungsten substrate, a substrate including tungsten foil, a flexible substrate, an attachment film, paper including a fibrous material, and a base material film. As an example of a glass substrate, a barium borosilicate glass substrate, an aluminoborosilicate glass substrate, a soda lime glass substrate, or the like can be given. Examples of a flexible substrate, an attachment film, a base material film, or the like are as follows: plastic typified by polyethylene terephthalate (PET), polyethylene naphthalate (PEN), and polyether sulfone (PES); a synthetic resin such as acrylic; polypropylene; polyester; polyvinyl fluoride; polyvinyl chloride; polyamide; polyimide; aramid; epoxy; an inorganic vapor deposition film; and paper. Specifically, the use of semiconductor substrates, single crystal substrates, SOI substrates, or the like enables the manufacture of small-sized transistors with a small variation in characteristics, size, shape, or the like and with high current capability. A circuit using such transistors achieves lower power consumption of the circuit or higher integration of the circuit.

A flexible substrate may be used as the substrate 102, and the transistor may be provided directly on the flexible substrate. Alternatively, a separation layer may be provided between the substrate 102 and the transistor. The separation layer can be used when part or the whole of a semiconductor device formed over the separation layer is separated from the substrate 102 and transferred onto another substrate. In such a case, the transistor can be transferred to a substrate having low heat resistance or a flexible substrate as well. For the above separation layer, a stack including inorganic films, which are a tungsten film and a silicon oxide film, or an organic resin film of polyimide or the like formed over a substrate can be used, for example.

Examples of a substrate to which the transistor is transferred include, in addition to the above-described substrates over which the transistor can be formed, a paper substrate, a cellophane substrate, an aramid film substrate, a polyimide film substrate, a stone substrate, a wood substrate, a cloth substrate (including a natural fiber (e.g., silk, cotton, and hemp), a synthetic fiber (e.g., nylon, polyurethane, and polyester), a regenerated fiber (e.g., acetate, cupra, rayon, or regenerated polyester), or the like), a leather substrate, a rubber substrate, and the like. When such a substrate is used, a transistor with excellent properties or a transistor with low power consumption can be formed, a device with high durability, high heat resistance can be provided, or reduction in weight or thickness can be achieved.

The insulating film 104 can be formed with a single layer or stacked layers using an oxide insulating film or a nitride insulating film. Note that at least a region of the insulating film 104 that is in contact with the oxide semiconductor film 106 is preferably formed using an oxide insulating film, in order to improve characteristics of the interface with the oxide semiconductor film 106. When the insulating film 104 is formed using an oxide insulating film from which oxygen is released by heating, oxygen contained in the insulating film 104 can be moved to the oxide semiconductor film 106 by heat treatment.

The thickness of the insulating film 104 can be greater than or equal to 50 nm, greater than or equal to 100 nm and less than or equal to 3000 nm, or greater than or equal to 200 nm and less than or equal to 1000 nm. By increasing the thickness of the insulating film 104, the amount of oxygen released from the insulating film 104 can be increased, and the interface state density at the interface between the insulating film 104 and the oxide semiconductor film 106 and oxygen vacancies in the region 106 d in the oxide semiconductor film 106 can be reduced.

The insulating film 104 can be formed to have a single-layer structure or a stacked-layer structure using, for example, any of silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, and Ga—Zn oxide.

The oxide semiconductor film 106 is typically formed using a metal oxide such as an In—Ga oxide, an In—Zn oxide, and an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf). Note that the oxide semiconductor film 106 has a light-transmitting property.

Note that in the case where the oxide semiconductor film 106 is formed using an In-M-Zn oxide, when summation of In and M is assumed to be 100 atomic %, the proportions of In and Mare preferably as follows: the proportion of In is greater than or equal to 25 atomic % and the proportion of M is less than 75 atomic %, or the proportion of In is greater than or equal to 34 atomic % and the proportion of M is less than 66 atomic %.

The energy gap of the oxide semiconductor film 106 is greater than or equal to 2 eV, greater than or equal to 2.5 eV or more, or greater than or equal to 3 eV.

The thickness of each of the oxide semiconductor film 106 can be greater than or equal to 3 nm and less than or equal to 200 nm, greater than or equal to 3 nm and less than or equal to 100 nm, or greater than or equal to 3 nm and less than or equal to 50 nm.

In the case where the oxide semiconductor film 106 contains an In-M-Zn oxide, the atomic ratio of the metal elements of a sputtering target used for forming the In-M-Zn oxide preferably satisfy In≧M and Zn≧M. The sputtering target preferably contains In, M, and Zn at an atomic ratio of 1:1:1, 1:1:1.2, 2:1:1.5, 2:1:2.3, 2:1:3, 3:1:2, or the like. Note that the proportion of each metal element in the atomic ratio of the oxide semiconductor film 106 to be formed varies within a range of ±40% of that in the above atomic ratio of the sputtering target as an error.

When silicon or carbon that is an element belonging to Group 14 is contained in the oxide semiconductor film 106, oxygen vacancies are increased in the oxide semiconductor film 106, and the oxide semiconductor film 106 become an n-type film. Thus, the concentration of silicon or carbon (the concentration measured by SIMS) in the oxide semiconductor film 106, particularly, the region 106 d, can be lower than or equal to 2×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁷ atoms/cm³. As a result, the transistor has positive threshold voltage (normally-off characteristics).

The concentration of alkali metal or alkaline earth metal in the oxide semiconductor film 106, particularly the region 106 d, which is measured by SIMS, can be lower than or equal to 1×10¹⁸ atoms/cm³, or lower than or equal to 2×10¹⁶ atoms/cm³. Alkali metal and alkaline earth metal might generate carriers when bonded to an oxide semiconductor, in which case the off-state current of the transistor might be increased. Therefore, it is preferable to reduce the concentration of alkali metal or alkaline earth metal in the region 106 d. As a result, the transistor has positive threshold voltage (normally-off characteristics).

When nitrogen is contained in the oxide semiconductor film 106, particularly the region 106 d, electrons serving as carriers are generated to increase the carrier density, so that the oxide semiconductor films 106 become an n-type film in some cases. Thus, a transistor including an oxide semiconductor film which contains nitrogen is likely to have normally-on characteristics. Therefore, nitrogen is preferably reduced as much as possible in the oxide semiconductor film, particularly the region 106 d. The nitrogen concentration, which is measured by SIMS, can be set to, for example, lower than or equal to 5×10¹⁸ atoms/cm³.

By reducing the impurity elements in the oxide semiconductor film 106, particularly the region 106 d, the carrier density of the oxide semiconductor film can be lowered. The oxide semiconductor film 106, particularly the region 106 d, can have a carrier density of 1×10¹⁷/cm³ or less, 1×10¹⁵/cm³ or less, 1×10¹³/cm³ or less, or 1×10¹¹/cm³ or less.

An oxide semiconductor film with a low impurity concentration and a low density of defect states is used for the oxide semiconductor film 106, in which case the transistor can have more excellent electrical characteristics. The state in which the impurity concentration is low and the density of defect states is low (the number of oxygen vacancies is small) is referred to as a “highly purified intrinsic” or “substantially highly purified intrinsic” state. A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier generation sources, and thus can have a low carrier density in some cases. Thus, a transistor including the oxide semiconductor film in which a channel region is formed is likely to have positive threshold voltage (normally-off characteristics). A highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has a low density of defect states and accordingly has few carrier traps in some cases. Furthermore, a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor film has an extremely low off-state current; the off-state current can be less than or equal to the measurement limit of a semiconductor parameter analyzer, i.e., less than or equal to 1×10⁻¹³ A, at a voltage (drain voltage) between a source electrode and a drain electrode of from 1 V to 10 V. Thus, the transistor whose channel region is formed in the oxide semiconductor film has small variations in electrical characteristics and high reliability in some cases.

The oxide semiconductor film 106 may have a non-single-crystal structure, for example. The non-single-crystal structure includes a c-axis aligned crystalline oxide semiconductor (CAAC-OS) which is described later, a polycrystalline structure, a microcrystalline structure which is described later, or an amorphous structure, for example. Among the non-single-crystal structure, the amorphous structure has the highest density of defect levels, whereas CAAC-OS has the lowest density of defect levels.

Note that the oxide semiconductor film 106 may be a mixed film including two or more of the following: a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a region of CAAC-OS described later, and a region having a single-crystal structure. The mixed film has a single-layer structure including, for example, two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single crystal structure in some cases. Furthermore, the mixed film has a stacked-layer structure of two or more of a region having an amorphous structure, a region having a microcrystalline structure, a region having a polycrystalline structure, a CAAC-OS region, and a region having a single-crystal structure in some cases.

Note that in the oxide semiconductor film 106, the region 106 b and the region 106 d differ in crystallinity in some cases. In the oxide semiconductor film 106, the region 106 c and the region 106 d differ in crystallinity in some cases. These cases are due to damage to the region 106 b or the region 106 c, which lowers their crystallinity, when the impurity elements are added to the region 106 b or the region 106 c.

The insulating film 106 can be formed to have a single-layer structure or a stacked-layer structure using an oxide insulating film or a nitride insulating film. Note that an oxide insulating film is preferably used as at least a region of the insulating film 108 that is in contact with the oxide semiconductor film 106, in order to improve characteristics of the interface with the oxide semiconductor film 106. The insulating film 108 may be formed to have a single-layer structure or a stacked-layer structure using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn oxide.

Furthermore, it is possible to prevent outward diffusion of oxygen from the oxide semiconductor film 106 and entry of hydrogen, water, and the like into the oxide semiconductor film 106 from the outside by providing an insulating film having a blocking effect against oxygen, hydrogen, water, and the like as the gate insulating film 108. Examples of the insulating film having a blocking effect against oxygen, hydrogen, water, and the like include aluminum oxide, aluminum oxynitride, gallium oxide, gallium oxynitride, yttrium oxide, yttrium oxynitride, hafnium oxide, and hafnium oxynitride.

The gate insulating film 108 may be formed using a high-k material such as hafnium silicate (HfSiO_(x)), hafnium silicate to which nitrogen is added (HfSi_(x)O_(y)N_(z)), hafnium aluminate to which nitrogen is added (HfAl_(x)O_(y)N_(z)), hafnium oxide, or yttrium oxide, so that gate leakage current of the transistor can be reduced.

By using an oxide insulating film that releases oxygen by being heated as the insulating film 108, oxygen contained in the insulating film 108 can be moved to the oxide semiconductor film 106 by heat treatment.

As the insulating film 108, a silicon oxynitride film with few defects can be used. In an ESR spectrum at 100 K or lower of the silicon oxynitride film with few defects, after heat treatment, a first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, a second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and a third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 are observed. The split width of the first and second signals and the split width of the second and third signals that are obtained by ESR measurement using an X-band are each approximately 5 mT. The sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and smaller than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and smaller than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and smaller than or equal to 1.966 is lower than 1×10¹⁸ spins/cm³, typically higher than or equal to 1×10¹⁷ spins/cm³ and lower than 1×10¹⁸ spins/cm³.

In the ESR spectrum at 100 K or lower, the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 correspond to signals attributed to nitrogen oxide (NO_(x); x is greater than or equal to 0 and less than or equal to 2, or greater than or equal to 1 and less than or equal to 2). Accordingly, the lower the sum of the spin densities of the first signal that appears at a g-factor of greater than or equal to 2.037 and less than or equal to 2.039, the second signal that appears at a g-factor of greater than or equal to 2.001 and less than or equal to 2.003, and the third signal that appears at a g-factor of greater than or equal to 1.964 and less than or equal to 1.966 is, the smaller the amount of nitrogen oxide contained in the silicon oxynitride film is.

In the silicon oxynitride film with few defects, the concentration of nitrogen which is measured by SIMS is lower than or equal to 6×10²⁰ atoms/cm³. When the insulating film 108 is formed using the silicon oxynitride film with few defects, nitrogen oxide is unlikely to be generated, so that the carrier traps at the interface between the oxide semiconductor film 106 and the insulating film 108 can be reduced. In addition, a shift in the threshold voltage of the transistor included in the semiconductor device can be reduced, which leads to a reduction in change in the threshold voltage of the transistor.

The thickness of the insulating film 108 can be greater than or equal to 5 nm and less than or equal to 400 nm, greater than or equal to 5 nm and less than or equal to 300 nm, or greater than or equal to 10 nm and less than or equal to 250 nm.

The conductive films 110, 112, and 114 are formed in the same process and thus contain the same material and have the same stacked-layer structure. The conductive films 110, 112, and 114 can be formed using a metal element selected from aluminum, chromium, copper, tantalum, titanium, molybdenum, nickel, iron, cobalt, and tungsten; an alloy containing any of these metal elements as a component; an alloy containing these metal elements in combination; or the like. Furthermore, one or more metal elements selected from manganese and zirconium may be used. The conductive films 110, 112, and 114 may have a single-layer structure or a stacked layer structure of two or more layers. For example, any of the following can be used: a single-layer structure of an aluminum film containing silicon; a single-layer structure of a copper film containing manganese; two-layer structure in which a titanium film is stacked over an aluminum film; a two-layer structure in which a titanium film is stacked over a titanium nitride film; a two-layer structure in which a tungsten film is stacked over a titanium nitride film; a two-layer structure in which a tungsten film is stacked over a tantalum nitride film or a tungsten nitride film; a two-layer structure in which a copper film is stacked over a copper film containing manganese; a three-layer structure in which a titanium film, an aluminum film, and a titanium film are stacked in this order; and a three-layer structure in which a copper film containing manganese, a copper film, and a copper film containing manganese are stacked in this order. Alternatively, an alloy film or a nitride film in which aluminum and one or more elements selected from titanium, tantalum, tungsten, molybdenum, chromium, neodymium, and scandium are combined may be used.

The conductive films 110, 112, and 114 can also be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide containing tungsten oxide, indium zinc oxide containing tungsten oxide, indium oxide containing titanium oxide, indium tin oxide containing titanium oxide, indium zinc oxide, or indium tin oxide containing silicon oxide. It is also possible to have a stacked-layer structure formed using the above light-transmitting conductive material and the above metal element.

The thickness of each of the conductive films 110, 112, and 114 can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.

The insulating film 116 can be formed to have a single-layer structure of a stacked-layer structure using an oxide insulating film or a nitride insulating film Note that an oxide insulating film is preferably used as at least a region of the insulating film 116 that is in contact with the oxide semiconductor film 106, in order to improve characteristics of the interface with the oxide semiconductor film 106. By using an oxide insulating film that releases oxygen by being heated as the insulating film 116, oxygen contained in the insulating film 116 can be moved to the oxide semiconductor film 106 by heat treatment.

The insulating film 116 can be formed to have a single-layer structure using, for example, silicon oxide, silicon oxynitride, silicon nitride oxide, silicon nitride, aluminum oxide, hafnium oxide, gallium oxide, or Ga—Zn oxide or a stacked-layer structure thereof.

The insulating film 118 is preferably a film serving as a barrier film against hydrogen, water, and the like from the outside. The insulating film 118 can be formed to have a single-layer structure using, for example, silicon nitride, silicon nitride oxide, or aluminum oxide or a stacked-layer structure thereof.

The thickness of each of the insulating films 116 and 118 can be greater than or equal to 30 nm and less than or equal to 500 nm, or greater than or equal to 100 nm and less than or equal to 400 nm.

<Structure 2 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 3A to 3C.

FIGS. 3A to 3C are a top view and cross-sectional views of a transistor 151 included in a semiconductor device. FIG. 3A is a top view of the transistor 151. FIG. 3B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 3A. FIG. 3C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 3A.

The transistor 151 illustrated in FIGS. 3A to 3C is characterized in that the conductive film 110, the conductive film 112, and the conductive film 114 each have a three-layer structure and in that the insulating film 104 has a stacked-layer structure of a nitride insulating film 104 a and an oxide insulating film 104 b. The other components are the same as those of the transistor 150, and the effect similar to that in the case of the transistor 150 is obtained.

First, the conductive films 110, 112, and 114 are described.

In the conductive film 110, a conductive film 110 a, a conductive film 110 b, and a conductive film 110 c are stacked in this order, and the conductive film 110 a and the conductive film 110 c cover surfaces of the conductive film 110 b. That is, the conductive film 110 a and the conductive film 110 c serve as protective films of the conductive film 110 b.

In the conductive film 112, a conductive film 112 a, a conductive film 112 b, and a conductive film 112 c are stacked in this order, and the conductive film 112 a and the conductive film 112 c cover surfaces of the conductive film 112 b, as in the case of the conductive film 110.

In the conductive film 114, a conductive film 114 a, a conductive film 114 b, and a conductive film 114 c are stacked in this order, and the conductive film 114 a and the conductive film 114 c cover surfaces of the conductive film 114 b, as in the case of the conductive film 110.

The conductive films 110 a, 112 a, and 114 a are formed using a material that prevents metal elements contained in the conductive films 110 b, 112 b, and 114 b from being diffused into the oxide semiconductor film 106. The conductive films 110 a, 112 a, and 114 a can be formed using titanium, tantalum, molybdenum, tungsten, an alloy of any of these, titanium nitride, tantalum nitride, molybdenum nitride, tantalum nitride, or the like. Alternatively, the conductive films 110 a, 112 a, and 114 a can be formed using a Cu—X alloy (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti) and the like.

In the case of using a Cu—X alloy (X is Mn, Ni, Cr, Fe, Co, Mo, Ta, or Ti), a covering film is formed in a region in contact with the oxide semiconductor film or a region in contact with an insulating film by heat treatment, in some cases. The covering film includes a compound containing X Examples of the compound containing X include an oxide of X, an In—X oxide, a Ga—X oxide, an In—Ga—X oxide, and In—Ga—Zn—X oxide. When the covering film is formed on surfaces of the conductive films 110 a, 112 a, and 114 a, the covering film functions as a blocking film, and Cu in the Cu—X alloy film can be prevented from entering the oxide semiconductor film.

Note that when the concentration of copper in a region serving as a channel in the oxide semiconductor film 106 is lower than or equal to 1×10¹⁸ atoms/cm³, the electron trap state density at the interface between the oxide semiconductor film 106 and the insulating film 108 serving as a gate insulating film can be reduced. As a result, a transistor having an excellent subthreshold swing (S value) can be manufactured.

The conductive films 110 b, 112 b, and 114 b are formed using a low-resistance material. The conductive film 110 b, the conductive film 112 b, and the conductive film 114 b can be formed using copper, aluminum, gold, silver, or the like, an alloy containing any of these, a compound containing any of these as a main component, or the like.

By forming the conductive films 110 c, 112 c, and 114 c using films in which metal elements contained in the conductive films 110 b, 112 b, and 114 b are passivated, the metal elements contained in the conductive films 110 b, 112 b, and 114 b can be prevented from moving to the oxide semiconductor film 106 in a step of forming the insulating film 116. The conductive films 110 c, 112 c, and 114 c can be formed using metal silicide, metal silicide-nitride, or the like; CuSi_(x) (x>0), CuSi_(x)N_(y) (x>0, y>0), and the like are typical examples thereof.

Here, a method for forming the conductive films 110 c, 112 c, and 114 c is described. Note that the conductive films 110 b, 112 b, and 114 b are formed using copper. The conductive film 110 c, the conductive film 112 c, and the conductive film 114 c are formed using CuSi_(x)N_(y) (x>0, y>0).

The conductive films 110 b, 112 b, and 114 b are exposed to plasma generated in a reducing atmosphere of hydrogen, ammonia, carbon monoxide, or the like to reduce oxide on surfaces of the conductive films 110 b, 112 b, and 114 b.

Next, the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b are exposed to silane while heating is performed at a temperature higher than or equal to 200° C. and lower than or equal to 400° C. Thus, copper contained in the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b acts as a catalyst, and the silane is decomposed into Si and H₂, and CuSi_(x) (x>0) is formed on the surfaces of the conductive films 110 b, 112 b, and 114 b.

Next, the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b are exposed to plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, so that CuSi_(x) (x>0) formed on the surfaces of the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b reacts with nitrogen contained in the plasma. In this manner, CuSi_(x)N_(y) (x>0, y>0) is formed as the conductive film 110 c, the conductive film 112 c, and the conductive film 114 c.

Note that in the above-described step, after the conductive films 110 b, 112 b, and 114 b are exposed to plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, the conductive films 110 b, 112 b, and 114 b are exposed to silane while heating is performed at a temperature higher than or equal to 200° C. and lower than or equal to 400° C., whereby CuSi_(x)N_(y) (x>0, y>0) may be formed as the conductive film 110 c, the conductive film 112 c, and the conductive film 114 c.

Next, the insulating film 104 in which the nitride insulating film 104 a and the oxide insulating film 104 b are stacked is described.

For example, the nitride insulating film 104 a can be formed using silicon nitride, silicon nitride oxide, aluminum nitride, aluminum nitride oxide, or the like. The oxide insulating film 104 b can be formed using silicon oxide, silicon oxynitride, aluminum oxide, or the like. Providing the nitride insulating film 104 a on the substrate 102 side can prevent hydrogen, water, and the like from the outside from being diffused into the oxide semiconductor film 106.

<Structure 3 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 4A to 4C, FIGS. 5A to 5C, and FIGS. 11A and 11B.

FIGS. 4A to 4C are a top view and cross-sectional views of a transistor 152 included in a semiconductor device. FIG. 4A is a top view of the transistor 152. FIG. 4B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 4A. FIG. 4C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 4A.

The transistor 152 illustrated in FIGS. 4A to 4C is characterized in that the oxide semiconductor film 106 has a multilayer structure. Specifically, the oxide semiconductor film 106 includes an oxide semiconductor film 107 a in contact with the insulating film 104, an oxide semiconductor film 107 b in contact with the oxide semiconductor film 107 a, and an oxide semiconductor film 107 c in contact with the oxide semiconductor film 107 b, the conductive film 110, the conductive film 112, the insulating film 108, and the insulating film 116. The other components are the same as those of the transistor 150 and the effect similar to that in the case of the transistor 150 is obtained.

The oxide semiconductor films 107 a, 107 b, and 107 c are typically formed using a metal oxide such as an In—Ga oxide, an In—Zn oxide, or an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf).

The oxide semiconductor films 107 a and 107 c are typically formed using an In—Ga oxide, an In—Zn oxide, an In—Mg oxide, a Zn—Mg oxide, or an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf), and has the energy at the bottom of the conduction band closer to a vacuum level than that of the oxide semiconductor film 107 b. Typically, the difference between the energy at the bottom of the conduction band of the oxide semiconductor film 107 b and the energy at the bottom of the conduction band of each of the oxide semiconductor film 107 a and the oxide semiconductor film 107 c is greater than or equal to 0.05 eV, greater than or equal to 0.07 eV, greater than or equal to 0.1 eV, or greater than or equal to 0.2 eV and also less than or equal to 2 eV, less than or equal to 1 eV, less than or equal to 0.5 eV, or less than or equal to 0.4 eV. Note that the difference between the vacuum level and the energy at the bottom of the conduction band is referred to as electron affinity. Note that the difference between the vacuum level and the energy at the bottom of the conduction band is referred to as electron affinity.

In the case where the oxide semiconductor film 107 b is formed using an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) and a target having an atomic ratio of In to M and Zn (In:M:Zn) of x₁:y₁:z₁ is used for forming the oxide semiconductor film 107 b, x₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6, and z₁/y₁ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₁/y₁ is greater than or equal to 1 and less than or equal to 6, a CAAC-OS film as the oxide semiconductor film 107 b is easily formed. Typical examples of the atomic ratio of In to M and Zn in the target are 1:1:1, 1:1:1.2, 2:1:1.5, 2:1:2.3, 2:1:3, and 3:1:2.

In the case where the oxide semiconductor films 107 a and 107 c are formed using an In-M-Zn oxide (M is Mg, Al, Ti, Ga, Y, Zr, La, Ce, Nd, or Hf) and a target having an atomic ratio of In to M and Zn (In:M:Zn) of x₂:y₂:z₂ is used for forming the oxide semiconductor film 107 a and the oxide semiconductor film 107 c, x₂/y₂ is preferably less than x₁/y₁, and z₂/y₂ is preferably greater than or equal to ⅓ and less than or equal to 6, further preferably greater than or equal to 1 and less than or equal to 6. Note that when z₂/y₂ is greater than or equal to 1 and less than or equal to 6, CAAC-OS films are easily formed as the oxide semiconductor film 107 a and the oxide semiconductor film 107 c. Typical examples of the atomic ratio of In to M and Zn in the target are 1:3:2, 1:3:4, 1:3:6, 1:3:8, 1:4:3, 1:4:4, 1:4:5, 1:4:6, 1:6:3, 1:6:4, 1:6:5, 1:6:6, 1:6:7, 1:6:8, and 1:6:9.

In the case where the oxide semiconductor films 107 a and 107 c are each formed using an In—Ga oxide, the oxide semiconductor films 107 a and 107 c can be formed by a sputtering method using an In—Ga metal oxide target (In:Ga=7:93). When the atomic ratio of In to Ga of an In—Ga oxide target, which is used to form the oxide semiconductor films 107 a and 107 c by a sputtering method using DC discharge, is x:y, it is preferable that y/(x+y) be less than or equal to 0.96, further preferably less than or equal to 0.95, for example, 0.93.

Note that the proportion of each metal element in the atomic ratio of each of the first oxide semiconductor films 107 a, 107 b, and 107 c varies within a range of ±40% of any of the above atomic ratios as an error.

The atomic ratio is not limited to the above, and the atomic ratio may be appropriately set in accordance with needed semiconductor characteristics.

The oxide semiconductor film 107 a and the oxide semiconductor film 107 c may have the same composition. For example, an In—Ga—Zn oxide in which the atomic ratio of In to Ga and Zn (In:Ga:Zn) is 1:3:2, 1:3:4, 1:4:5, 1:4:6, 1:4:7, or 1:4:8 may be used for the oxide semiconductor film 107 a and the oxide semiconductor film 107 c.

Alternatively, the oxide semiconductor film 107 a and the oxide semiconductor film 107 c may have different compositions. For example, an In—Ga—Zn oxide in which the atomic ratio of In to Ga and Zn (In:Ga:Zn) is 1:3:2 may be used for the oxide semiconductor film 107 a and an In—Ga—Zn oxide in which the atomic ratio of In to Ga and Zn (In:Ga:Zn) is 1:3:4 or 1:4:5 may be used for the oxide semiconductor film 107 c.

The thickness of each of the oxide semiconductor film 107 a and the oxide semiconductor film 107 c is greater than or equal to 3 nm and less than or equal to 100 nm, or greater than or equal to 3 nm and less than or equal to 50 nm. The thickness of the oxide semiconductor film 107 b is greater than or equal to 3 nm and less than or equal to 200 nm, greater than or equal to 3 nm and less than or equal to 100 nm, or greater than or equal to 3 nm and less than or equal to 50 nm. Note that when the thickness of each of the oxide semiconductor film 107 a and the oxide semiconductor film 107 c is smaller than the thickness of the oxide semiconductor film 107 b, the amount of change in the threshold voltage of the transistor can be reduced.

The interface between the oxide semiconductor film 107 a and the oxide semiconductor film 107 b and the interface between the oxide semiconductor film 107 b and the oxide semiconductor film 107 c can be observed by scanning transmission electron microscopy (STEM) in some cases.

Any of the crystal structures of the oxide semiconductor film 106 described above can be used as appropriate for the oxide semiconductor films 107 a, 107 b, and 107 c.

The oxide semiconductor film 107 a and the oxide semiconductor film 107 c in each of which oxygen vacancies are less likely to be generated than in the oxide semiconductor film 107 b are provided in contact with an upper surface and a lower surface of the oxide semiconductor film 107 b, whereby oxygen vacancies in the oxide semiconductor film 107 b can be reduced. Furthermore, because the oxide semiconductor film 107 b is in contact with the oxide semiconductor film 107 a and the oxide semiconductor film 107 c that contain one or more metal elements forming the oxide semiconductor film 107 b, the densities of interface levels at the interface between the oxide semiconductor film 107 a and the oxide semiconductor film 107 b and at the interface between the oxide semiconductor film 107 b and the oxide semiconductor film 107 c are extremely low. Accordingly, oxygen vacancies in the oxide semiconductor film 107 b can be reduced.

Here, in the case where the oxide semiconductor film 107 b is in contact with an insulating film containing a different constituent element (e.g., an insulating film containing a silicon oxide film), an interface state is sometimes formed at the interface between the two films and the interface state forms a channel. At this time, another transistor having a different threshold voltage appears, so that an apparent threshold voltage of the transistor is varied. However, because the oxide semiconductor film 107 a containing one or more kinds of metal elements forming the oxide semiconductor film 107 b is in contact with the oxide semiconductor film 107 b, an interface state is unlikely to be formed at the interface between the oxide semiconductor film 107 a and the oxide semiconductor film 107 b. Thus, providing the oxide semiconductor film 107 a makes it possible to reduce variations in the electrical characteristics of the transistor, such as threshold voltage.

In the case where a channel is formed at the interface between the insulating film 108 and the oxide semiconductor film 107 b, interface scattering occurs at the interface, so that the field-effect mobility of a transistor is reduced. However, because the oxide semiconductor film 107 c containing one or more kinds of metal elements forming the oxide semiconductor film 107 b is provided in contact with the oxide semiconductor film 107 b, scattering of carriers does not easily occur at the interface between the oxide semiconductor film 107 b and the oxide semiconductor film 107 c, so that the field-effect mobility of the transistor can be increased.

The oxide semiconductor film 107 a and the oxide semiconductor film 107 c each also serve as a barrier film that suppresses formation of an impurity state due to the entry of the constituent elements of the insulating films 104 and 108 or the constituent elements of the conductive films 110 and 112 into the oxide semiconductor film 107 b.

For example, in the case of using an insulating film containing silicon or an insulating film containing carbon as the insulating film 104 and the insulating film 108, silicon in the insulating film 104 and the insulating film 108 or carbon mixed in the insulating film 104 and the insulating film 108 enters the oxide semiconductor film 107 a and the oxide semiconductor film 107 c to a depth of about several nanometers from the interfaces in some cases. An impurity such as silicon or carbon entering the oxide semiconductor film 107 b forms an impurity state. The impurity state serves as a donor and generates an electron, so that the oxide semiconductor film 107 b might become n-type.

However, when the thicknesses of the oxide semiconductor films 107 a and 107 c are larger than several nanometers, the impurity such as silicon or carbon that has entered the oxide semiconductor films does not reach the oxide semiconductor film 107 b, so that the influence of impurity states is reduced

Thus, the transistor described in this embodiment is a transistor in which variations in the electrical characteristics such as threshold voltage are reduced.

FIGS. 5A to 5C illustrate a transistor having a structure different from that illustrated in FIGS. 4A to 4C.

FIGS. 5A to 5C are a top view and cross-sectional views of a transistor 153 included in a semiconductor device. FIG. 5A is a top view of the transistor 153, FIG. 5B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 5A, and FIG. 5C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 5A.

As in the transistor 153 illustrated in FIGS. 5A to 5C, the oxide semiconductor film 106 may have a stacked-layer structure including the oxide semiconductor film 107 b in contact with the insulating film 104 and the oxide semiconductor film 107 c in contact with the oxide semiconductor film 107 b and the insulating film 108. The other components are the same as those of the transistor 150 and the effect similar to that in the case of the transistor 150 is obtained.

<Band Structure>

Here, band structures of the transistors illustrated in FIGS. 4A to 4C and FIGS. 5A to 5C are described. Note that FIG. 11A is a band structure of the transistor 153 illustrated in FIGS. 4A to 4C; for easy understanding, energy (Ec) of the bottom of the conduction band of each of the insulating film 104, the oxide semiconductor film 107 a, the oxide semiconductor film 107 b, the oxide semiconductor film 107 c, and the insulating film 108 is shown. FIG. 11B is a band structure of the transistor 154 illustrated in FIGS. 5A to 5C; for easy understanding, energy (Ec) of the bottom of the conduction band of each of the insulating film 104, the oxide semiconductor film 107 b, the oxide semiconductor film 107 c, and the insulating film 108 is shown.

As shown in FIG. 11A, the energy at the bottom of the conduction band changes continuously in the oxide semiconductor film 107 a, the oxide semiconductor film 107 b, and the oxide semiconductor film 107 c. This can be understood also from the fact that the constituent elements are common among the oxide semiconductor film 107 a, the oxide semiconductor film 107 b, and the oxide semiconductor film 107 c and oxygen easily diffuses among the oxide semiconductor films 107 a, 107 b, and 107 c. Thus, the oxide semiconductor films 107 a, 107 b, and 107 c have a continuous physical property although they are a stack of films having different compositions.

The oxide semiconductor films that are stacked and contain the same main components have not only a simple stacked-layer structure of the layers but also a continuous energy band (here, in particular, a well structure having a U shape in which the bottom of the conduction band continuously varies among the layers (U-shape well)). That is, a stacked-layer structure is formed so that impurities that cause defect levels such as a trap center or a recombination center for the oxide semiconductor or impurities that inhibit carrier flow do not exist at the interfaces between the layers. If impurities are mixed between the stacked oxide semiconductor films, the continuity of the energy band is lost and carriers disappear by a trap or recombination.

Note that FIG. 11A shows the case where the Ec of the oxide semiconductor film 107 a and the Ec of the oxide semiconductor film 107 c are equal to each other; however, they may be different from each other.

As shown in FIG. 11A, the oxide semiconductor film 107 b serves as a well and a channel of the transistor 152 is formed in the oxide semiconductor film 107 b. Note that because the energy at the bottom of the conduction band of the oxide semiconductor films 107 a, 107 b, and 107 c changes continuously, a channel having a U-shaped well structure can also be referred to as a buried channel.

As shown in FIG. 11B, the energy at the bottom of the conduction band may change continuously in the oxide semiconductor film 107 b and the oxide semiconductor film 107 c.

As shown in FIG. 11B, the oxide semiconductor film 107 b serves as a well and a channel of the transistor 153 is formed in the oxide semiconductor film 107 b.

In the transistor 152 illustrated in FIGS. 4A to 4C, the oxide semiconductor film 107 a and the oxide semiconductor film 107 c contain one or more kinds of metal elements that form the oxide semiconductor film 107 b. Therefore, an interface state is not easily formed at the interface between the oxide semiconductor film 107 a and the oxide semiconductor film 107 b and at the interface between the oxide semiconductor film 107 c and the oxide semiconductor film 107 b. Thus, providing the oxide semiconductor film 107 a and the oxide semiconductor film 107 c makes it possible to reduce variations or changes in electrical characteristics of the transistor, such as threshold voltage.

In the transistor 153 illustrated in FIGS. 5A to 5C, the oxide semiconductor film 107 c contain one or more kinds of metal elements that form the oxide semiconductor film 107 b. Therefore, an interface state is not easily formed at the interface between the oxide semiconductor film 107 c and the oxide semiconductor film 107 b. Thus, providing the oxide semiconductor film 107 c makes it possible to reduce variations or changes in electrical characteristics of the transistor, such as threshold voltage.

<Structure 4 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 6A to 6D.

FIGS. 6A to 6D are a top view and cross-sectional views of a transistor 154 included in a semiconductor device. FIG. 6A is a top view of the transistor 154. FIGS. 6B and 6D are cross-sectional views taken along the dashed-dotted line Y1-Y2 in FIG. 6A. FIG. 6C and is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 6A.

The transistor 154 illustrated in FIGS. 6A to 6D is characterized in that the conductive film 120 is provided so as to be overlapped with the oxide semiconductor film 106 with the insulating film 104 therebetween. That is, the conductive film 120 serves as a gate electrode. Furthermore, the transistor 154 is a transistor having a dual-gate structure. The other components are the same as those of the transistor 150, and the effect similar to that in the case of the transistor 150 is obtained.

The threshold voltage of the transistor 154 can be controlled by supplying different potentials to the conductive film 114 and the conductive film 120 that are not connected to each other. FIG. 6D is an example of a cross-sectional structure in that case. Note that in the case where the conductive films 114 and 120 are not connected to each other, one of the conductive films may be supplied with a signal for turning a transistor on and off and the other may be supplied with a constant potential. Alternatively, by supplying the same potential to the conductive film 114 and the conductive film 120 that are connected to each other as illustrated in FIG. 6B, variations in the initial characteristics can be reduced, and degradation of the transistor due to a negative gate bias-temperature (−GBT) stress test and a change in the rising voltage of the on-state current at different drain voltages can be suppressed. In addition, a region where carriers flow through the oxide semiconductor film 106 becomes larger in the film thickness direction, so that the amount of carrier movement is increased. As a result, the on-state current and field-effect mobility of the transistor 154 are increased. When the channel length of the transistor is less than 2.5 μm, preferably greater than or equal to 1.45 μm and less than or equal to 2.2 μm, the on-state current can be further increased and the field-effect mobility can be increased.

Note that a structure in which the conductive film 120 and the conductive films 110 and 112 do not overlap each other may be employed. FIG. 39A illustrates an example of the structure. When the conductive films 114 and 120 are connected to each other, noise cannot easily enter from the conductive film 120.

Alternatively, a structure in which the conductive film 120 and the conductive films 110 and 112 overlap each other may be employed. FIG. 39B illustrates an example of the structure. When the conductive films 114 and 120 are not connected to each other and a constant potential is supplied to the conductive film 120, a potential can be supplied to the whole of the oxide semiconductor film 106.

<Method 1 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 150 illustrated in FIGS. 1A to 1C is described with reference to FIGS. 7A to 7D, FIGS. 8A to 8D, and FIGS. 9A and 9B.

The films included in the transistor 150 (i.e., the insulating film, the oxide semiconductor film, the conductive film, and the like) can be formed by any of a sputtering method, a chemical vapor deposition (CVD) method, a vacuum evaporation method, and a pulsed laser deposition (PLD) method. Alternatively, a coating method or a printing method can be used. Although the sputtering method and a plasma-enhanced chemical vapor deposition (PECVD) method are typical examples of the film formation method, a thermal CVD method may be used. As the thermal CVD method, a metal organic chemical vapor deposition (MOCVD) method or an atomic layer deposition (ALD) method may be used, for example.

Deposition by a thermal CVD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, and a source gas and an oxidizer are supplied to the chamber at a time and react with each other in the vicinity of the substrate or over the substrate. Thus, no plasma is generated in the deposition; therefore, the thermal CVD method has an advantage that no defect due to plasma damage is caused.

Deposition by the ALD method may be performed in such a manner that the pressure in a chamber is set to an atmospheric pressure or a reduced pressure, source gases for reaction are sequentially introduced into the chamber, and then the sequence of the gas introduction is repeated. For example, two or more kinds of source gases are sequentially supplied to the chamber by switching respective switching valves (also referred to as high-speed valves). For example, a first source gas is introduced, an inert gas (e.g., argon or nitrogen) or the like is introduced at the same time as or after the introduction of the first source gas so that the source gases are not mixed, and then a second source gas is introduced. Note that in the case where the first source gas and the inert gas are introduced at a time, the inert gas serves as a carrier gas, and the inert gas may also be introduced at the same time as the introduction of the second source gas. Alternatively, the first source gas may be exhausted by vacuum evacuation instead of the introduction of the inert gas, and then the second source gas may be introduced. The first source gas is adsorbed on the surface of the substrate to form a first single-atomic layer; then the second source gas is introduced to react with the first single-atomic layer; as a result, a second single-atomic layer is stacked over the first single-atomic layer, so that a thin film is formed.

The sequence of the gas introduction is repeated plural times until a desired thickness is obtained, whereby a thin film with excellent step coverage can be formed. The thickness of the thin film can be adjusted by the number of repetition times of the sequence of the gas introduction; therefore, an ALD method makes it possible to accurately adjust a thickness and thus is suitable for manufacturing a minute FET.

As illustrated in FIG. 7C, the insulating film 104 is formed over the substrate 102.

The insulating film 104 can be formed by a sputtering method, a CVD method, an evaporation method, a pulsed laser deposition (PLD) method, a printing method, a coating method, or the like, as appropriate. The insulating film 104 can be formed in such a manner that, after an insulating film is formed over the substrate 102, oxygen is added to the insulating film to form the insulating film 104. Examples of the oxygen that is added to the insulating film include an oxygen radical, an oxygen atom, an oxygen atomic ion, an oxygen molecular ion, and the like. As a method for adding the oxygen, an ion doping method, an ion implantation method, plasma treatment, or the like can be given. Alternatively, after a film which suppresses release of oxygen is formed over the insulating film, oxygen may be added to the insulating film through the film.

As the insulating film 104, a silicon oxide film or a silicon oxynitride film from which oxygen can be released by heat treatment can be forming under the following conditions: the substrate placed in a treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., or higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, or greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², or greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

Here, a method in which a film which suppresses release of oxygen is formed over the insulating film and then oxygen is added to the insulating film through the film is described with reference to FIGS. 7A and 7B.

As illustrated in FIG. 7A, an insulating film 103 is formed over the substrate 102.

Next, as illustrated in FIG. 7B, a film 119 that suppresses release of oxygen is formed over the insulating film 103. Then, oxygen 121 is added to the insulating film 103 through the film 119.

The film 119 which suppresses release of oxygen is formed using any of the following conductive materials: a metal element selected from aluminum, chromium, tantalum, titanium, molybdenum, nickel, iron, cobalt, and tungsten; an alloy containing the above-described metal element as a component; an alloy containing any of the above-described metal elements in combination; a metal nitride containing the above-described metal element; a metal oxide containing the above-described metal element; a metal nitride oxide containing the above-described metal element; and the like.

The thickness of the film 119 that suppresses release of oxygen can be greater than or equal to 1 nm and less than or equal to 20 nm, or greater than or equal to 2 nm and less than or equal to 10 nm.

As a method for adding the oxygen 121 to the insulating film 103 through the film 119, an ion doping method, an ion implantation method, plasma treatment, or the like is given. By adding oxygen to the insulating film 103 with the film 119 provided over the insulating film 103, the film 119 serves as a protective film which suppresses release of oxygen from the insulating film 103. Thus, a larger amount of oxygen can be added to the insulating film 103.

In the case where oxygen is added by plasma treatment, by making oxygen excited by a microwave to generate high density oxygen plasma, the amount of oxygen added to the oxide insulating film 103 can be increased.

After that, the film 119 is removed; consequently, the insulating film 104 to which oxygen is added can be formed over the substrate 102 as illustrated in FIG. 7C. Note that the treatment for adding oxygen that is illustrated in FIG. 7B is not necessarily performed as long as the insulating film 104 contains a sufficient amount of oxygen after being formed.

Next, as illustrated in FIG. 7D, the oxide semiconductor film 106 is formed over the insulating film 104. Next, the insulating film 108 is formed over the insulating film 104 and the oxide semiconductor film 106.

A formation method of the oxide semiconductor film 106 is described below. An oxide semiconductor film is formed over the insulating film 104 by a sputtering method, a coating method, a pulsed laser deposition method, a laser ablation method, a thermal CVD method, or the like. Then, after a mask is formed over the oxide semiconductor film by a lithography process, the oxide semiconductor film is partly etched using the mask. Thus, the oxide semiconductor film 106 can be formed as illustrated in FIG. 7D. After that, the mask is removed. Note that heat treatment may be performed after the oxide semiconductor film 106 is formed by etching part of the oxide semiconductor film.

Alternatively, by using a printing method for forming the oxide semiconductor film 106, the oxide semiconductor film 106 subjected to element isolation can be formed directly

As a power supply device for generating plasma in the case of forming the oxide semiconductor film by a sputtering method, an RF power supply device, an AC power supply device, a DC power supply device, or the like can be used as appropriate. Note that a CAAC-OS film can be formed using an AC power supply device or a DC power supply device. In forming the oxide semiconductor film, a sputtering method using an AC power supply device or a DC power supply device is preferable to a sputtering method using an RF power supply device because the oxide semiconductor film can be uniform in film thickness, film composition, or crystallinity.

As a sputtering gas, a rare gas (typically argon), an oxygen gas, or a mixed gas of a rare gas and oxygen is used as appropriate. In the case of using the mixed gas of a rare gas and oxygen, the proportion of oxygen to a rare gas is preferably increased.

A target may be selected as appropriate in accordance with the composition of an oxide semiconductor film to be formed.

For example, in the case where the oxide semiconductor film is formed by a sputtering method at a substrate temperature higher than or equal to 150° C. and lower than or equal to 750° C., higher than or equal to 150° C. and lower than or equal to 450° C., or higher than or equal to 200° C. and lower than or equal to 350° C., a CAAC-OS film can be formed. In the case where the substrate temperature is higher than or equal to 25° C. and lower than 150° C., a microcrystalline oxide semiconductor film can be formed.

For the deposition of the CAAC-OS film to be described later, the following conditions are preferably used.

By suppressing entry of impurities during the deposition, the crystal state can be prevented from being broken by the impurities. For example, the concentration of impurities (e.g., hydrogen, water, carbon dioxide, and nitrogen) which exist in the deposition chamber may be reduced. The concentration of impurities in a deposition gas may be reduced. Specifically, a deposition gas whose dew point is −80° C. or lower, or −100° C. or lower is used.

Furthermore, it is preferable that the proportion of oxygen in the deposition gas be increased and the power be optimized in order to reduce plasma damage at the deposition. The proportion of oxygen in the deposition gas is 30 vol. % or higher, or 100 vol. %.

After the oxide semiconductor film is formed, dehydrogenation or dehydration may be performed by heat treatment. The heat treatment is performed typically at a temperature higher than or equal to 150° C. and lower than the strain point of the substrate, higher than or equal to 250° C. and lower than or equal to 450° C., or higher than or equal to 300° C. and lower than or equal to 450° C.

The heat treatment is performed under an inert gas atmosphere containing nitrogen or a rare gas such as helium, neon, argon, xenon, or krypton. Alternatively, the heat treatment may be performed under an inert gas atmosphere first, and then under an oxygen atmosphere. It is preferable that the above inert gas atmosphere and the above oxygen atmosphere do not contain hydrogen, water, and the like. The treatment time is from 3 minutes to 24 hours.

An electric furnace, an RTA apparatus, or the like can be used for the heat treatment. With the use of an RTA apparatus, the heat treatment can be performed at a temperature of higher than or equal to the strain point of the substrate if the heating time is short. Therefore, the heat treatment time can be shortened.

By forming the oxide semiconductor film while it is heated or performing heat treatment after the formation of the oxide semiconductor film, the hydrogen concentration in the oxide semiconductor film, which is measured by SIMS, can be 5×10¹⁹ atoms/cm³ or lower, 1×10¹⁹ atoms/cm³ or lower, 5×10¹⁸ atoms/cm³ or lower, 1×10¹⁸ atoms/cm³ or lower, 5×10¹⁷ atoms/cm³ or lower, or 1×10¹⁶ atoms/cm³ or lower.

In the case where an oxide semiconductor film, for example, an In—Ga—Zn—O film is formed with a deposition apparatus employing ALD, an In(CH₃)₃ gas and an O₃ gas are sequentially introduced plural times to form an In—O layer, a Ga(CH₃)₃ gas and an O₃ gas are introduced at a time to form a Ga—O layer, and then a Zn(CH₃)₂ gas and an O₃ gas are introduced at a time to form a Zn—O layer. Note that the order of these layers is not limited to this example. A mixed compound layer such as an InGaO₂ layer, an InZnO₂ layer, a GaInO layer, a ZnInO layer, or a GaZnO layer may be formed by mixing of these gases. Note that although an H₂O gas which is obtained by bubbling with an inert gas such as Ar may be used instead of an O₃ gas, it is preferable to use an O₃ gas, which does not contain H. Instead of an In(CH₃)₃ gas, an In(C₂H₅)₃ may be used. Instead of a Ga(CH₃)₃ gas, a Ga(C₂H₅)₃ gas may be used. Alternatively, a Zn(CH₃)₂ gas may be used.

Here, an oxide semiconductor film having a thickness of 35 nm is formed by a sputtering method, and then, heat treatment is performed so that oxygen contained in the insulating film 104 is moved to the oxide semiconductor film. Next, a mask is formed over the oxide semiconductor film, and part of the oxide semiconductor film is selectively etched. In this manner, the oxide semiconductor film 106 is forming.

When the heat treatment is performed at a temperature higher than 350° C. and lower than or equal to 650° C., or higher than or equal to 450° C. and lower than or equal to 600° C., it is possible to obtain an oxide semiconductor film whose proportion of CAAC, which is described later, is greater than or equal to 60% and less than 100%, greater than or equal to 80% and less than 100%, greater than or equal to 90% and less than 100%, or greater than or equal to 95% and less than or equal to 98%. Furthermore, it is possible to obtain an oxide semiconductor film having a low content of hydrogen, water, and the like. That is, an oxide semiconductor film with a low impurity concentration and a low density of defect states can be formed.

The insulating film 108 can be formed by the formation method of the insulating film 104 as appropriate.

As the insulating film 108, a silicon oxide film or a silicon oxynitride film can be formed by a CVD method. In this case, a deposition gas containing silicon and an oxidizing gas are preferably used as a source gas. Typical examples of the deposition gas containing silicon include silane, disilane, trisilane, and silane fluoride. As the oxidizing gas, oxygen, ozone, dinitrogen monoxide, and nitrogen dioxide can be given as examples.

A silicon oxynitride film with few defects can be formed as the insulating film 108 by a CVD method under the conditions where the ratio of an oxidizing gas to a deposition gas is higher than 20 times and lower than 100 times, or higher than or equal to 40 times and lower than or equal to 80 times and the pressure in a treatment chamber is lower than 100 Pa, or lower than or equal to 50 Pa.

A silicon oxide film or a silicon oxynitride film that is dense can be formed as the insulating film 108 under the following conditions: the substrate placed in a treatment chamber of a plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 280° C. and lower than or equal to 400° C., the pressure in the treatment chamber is greater than or equal to 20 Pa and less than or equal to 250 Pa, preferably greater than or equal to 100 Pa and less than or equal to 250 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power is supplied to an electrode provided in the treatment chamber.

The insulating film 108 can be formed by a plasma CVD method using a microwave. The microwave refers to a wave in the frequency range of 300 MHz to 300 GHz. In a microwave, electron temperature is low and electron energy is low. Furthermore, in supplied power, the proportion of power used for acceleration of electrons is low, and therefore, much more power can be used for dissociation and ionization of molecules. Thus, plasma with high density (high-density plasma) can be excited. Therefore, the insulating film 104 and the oxide semiconductor film 106, and a deposit are less damaged by plasma, and the insulating film 108 with few defects can be formed.

Alternatively, the insulating film 108 can be formed by a CVD method using an organosilane gas. As the organosilane gas, any of the following silicon-containing compound can be used: tetraethyl orthosilicate (TEOS) (chemical formula: Si(OC₂H₅)₄); tetramethylsilane (TMS) (chemical formula: Si(CH₃)₄); tetramethylcyclotetrasiloxane (TMCTS); octamethylcyclotetrasiloxane (OMCTS); hexamethyldisilazane (HMDS); triethoxysilane (SiH(OC₂H₅)₃); trisdimethylaminosilane (SiH(N(CH₃)₂)₃); and the like. The insulating film 108 having high coverage can be formed by a CVD method using an organosilane gas.

In the case where a gallium oxide film is formed as the insulating film 108, a metal organic chemical vapor deposition (MOCVD) method can be used.

In the case where a hafnium oxide film is formed as the insulating film 108 by a thermal CVD method such as an MOCVD method or an ALD method, two kinds of gases, i.e., ozone (O₃) as an oxidizer and a source gas which is obtained by vaporizing a liquid containing a solvent and a hafnium precursor compound (a hafnium alkoxide solution, which is typified by tetrakis(dimethylamide)hafnium (TDMAH)), are used. Note that the chemical formula of tetrakis(dimethylamide)hafnium is Hf[N(CH₃)₂]₄. Examples of another material liquid include tetrakis(ethylmethylamide)hathium.

In the case where an aluminum oxide film is formed as the insulating film 108 by a thermal CVD method such as an MOCVD method or an ALD method, two kinds of gases, i.e., H₂O as an oxidizer and a source gas which is obtained by vaporizing a liquid containing a solvent and an aluminum precursor compound (e.g., trimethylaluminum (TMA)) are used. Note that the chemical formula of trimethylaluminum is Al(CH₃)₃. Examples of another material liquid include tris(dimethylamide)aluminum, triisobutylaluminum, and aluminum tris(2,2,6,6-tetramethyl-3,5-heptanedionate). Note that the ALD method enables the insulating film 108 to have excellent coverage and small thickness.

In the case where a silicon oxide film is formed as the insulating film 108 by a thermal CVD method such as an MOCVD method or an ALD method, hexachlorodisilane is adsorbed on the insulating film 104 and the oxide semiconductor film 106, chlorine contained in adsorbate is removed, and radicals of an oxidizing gas (e.g., O₂ or dinitrogen monoxide) are supplied to react with the adsorbate.

Here, a silicon oxynitride film is formed as the insulating film 108 by a plasma CVD method.

Next, after a mask is formed over the insulating film 108 by lithography, the insulating film 108 is partly etched to form the opening portion 140 a and the opening portion 140 b through which parts of the oxide semiconductor film 106 are exposed, as illustrated in FIG. 8A.

As a method for etching the insulating film 108, a wet etching method and/or a dry etching method can be employed as appropriate.

Next, as illustrated in FIG. 8B, the conductive film 109 is formed over the oxide semiconductor film 106 and the insulating film 108.

In the case where the conductive film 109 is formed using, for example, a low-resistance material, entry of the low-resistance material into the oxide semiconductor film leads to poor electrical characteristics of the transistor. In this embodiment, the insulating film 108 is formed before the conductive film 109 is formed; thus, a channel in the oxide semiconductor film 106 is not in contact with the conductive film 109. Therefore, variations in the electrical characteristics, typically threshold voltage, of the transistor can be suppressed.

The conductive film 109 can be formed by a sputtering method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, a thermal CVD method, or the like.

Alternatively, a tungsten film can be formed for the conductive film 109 with a deposition apparatus employing ALD. In that case, a WF₆ gas and a B₂H₆ gas are sequentially introduced more than once to form an initial tungsten film, and then a WF₆ gas and an H₂ gas are introduced at a time, so that a tungsten film is formed. Note that an SiH₄ gas may be used instead of a B₂H₆ gas.

Next, as illustrated in FIG. 8C, a mask 111 is formed over the conductive film 109 by lithography, and then the conductive film 109 is exposed to an etchant and/or an etching gas 123, whereby the conductive films 110, 112, and 114 are formed. Note that since the conductive films 110, 112, and 114 are formed by processing the conductive film 109, the conductive films 110, 112, and 114 contain the same metal, i.e., the same metal element as the conductive film 109.

As a method for etching the conductive film 109, a wet etching method and/or a dry etching method can be employed as appropriate. Note that after the conductive film 109 is etched, a cleaning step for removing a residue on a side surface of the insulating film 108 may be performed. As a result, leakage current between the conductive film 114 serving as a gate electrode and the oxide semiconductor film 106 can be reduced.

Note that the conductive films 110, 112, and 114 may be formed by an electrolytic plating method, a printing method, an inkjet method, or the like instead of the above formation method.

Next, as illustrated in FIG. 8D, an impurity element 117 is added to the oxide semiconductor film 106 with the mask 111 left. As a result, the impurity element is added to regions that are not covered with the mask 111 in the oxide semiconductor film. Note that by the addition of the impurity element 117, oxygen vacancies are formed in the oxide semiconductor film 106.

As a method for adding the impurity element 117, an ion doping method, an ion implantation method, plasma treatment, or the like can be given. In the case of plasma treatment, plasma is generated in a gas atmosphere containing an impurity element to be added and plasma treatment is performed, whereby the impurity element can be added. A dry etching apparatus, a plasma CVD apparatus, a high-density plasma CVD apparatus, or the like can be used to generate the plasma.

Note that, as a source gas of the impurity element 117, one or more of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, H₂, and a rare gas can be used. Alternatively, one or more of B₂H₆, PH₃, N₂, NH₃, AlH₃, AlCl₃, F₂, HF, and H₂ which are diluted with a rare gas can be used. By adding the impurity element 117 to the oxide semiconductor film 106 using one or more of B₂H₆, PH₃, N₂, NH₃, AlH₃, AlCl₃, F₂, HF, and H₂ that are diluted with a rare gas, the rare gas and one or more of hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorine can be added at a time to the oxide semiconductor film 106.

Alternatively, after a rare gas is added to the oxide semiconductor film 106, one or more of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, and H₂ may be added to the oxide semiconductor film 106.

Further alternatively, after one or more of B₂H₆, PH₃, CH₄, N₂, NH₃, AlH₃, AlCl₃, SiH₄, Si₂H₆, F₂, HF, and H₂ are added to the oxide semiconductor film 106, a rare gas may be added to the oxide semiconductor film 106.

The addition of the impurity element 117 is controlled by appropriately setting the implantation conditions such as the acceleration voltage and the dose. For example, in the case where argon is added by an ion implantation method, the acceleration voltage is set to 10 kV and the dose is set to greater than or equal to 1×10¹³ ions/cm² and less than or equal to 1×10¹⁶ ions/cm², e.g., 1×10¹⁴ ions/cm². In the case where a phosphorus ion is added by an ion implantation method, the acceleration voltage is set to 30 kV and the dose is set to greater than or equal to 1×10¹³ ions/cm² and less than or equal to 5×10¹⁶ ions/cm², e.g., 1×10¹⁵ ions/cm².

FIGS. 10A to 10C are schematic views in the thickness direction illustrating regions of the oxide semiconductor film 106 to which the impurity element 117 is added. FIGS. 10A to 10C are enlarged views of the oxide semiconductor film 106 and the vicinity thereof.

As illustrated in FIG. 10A, a region to which the impurity element 117 is added is formed in the insulating film 104, the oxide semiconductor film 106, and the insulating film 108 in some cases. Note that an end portion 135 of the region to which the impurity element 117 is added is positioned inside the insulating film 104 in a depth direction of a region where the oxide semiconductor film 106 is exposed. Note that the depth direction refers to a direction from the insulating film 108 to the insulating film 104 that is parallel to the thickness direction of the oxide semiconductor film 106.

As illustrated in FIG. 10B, a region to which the impurity element 117 is added is formed in the oxide semiconductor film 106 and the insulating film 108 in some cases. Note that an end portion 136 of the region to which the impurity element 117 is added is positioned at the interface between the insulating film 104 and the oxide semiconductor film 106 in a depth direction of a region where the oxide semiconductor film 106 is exposed.

As illustrated in FIG. 10C, a region to which the impurity element 117 is added is formed in the oxide semiconductor film 106 and the insulating film 108 in some cases. Note that an end portion 137 of the region to which the impurity element 117 is added is positioned inside the oxide semiconductor film 106 in a depth direction of a region where the oxide semiconductor film 106 is exposed.

As a result, low-resistance regions can be formed in the oxide semiconductor film 106. Specifically, the region 106 b and the region 106 c illustrated in FIG. 2 can be formed. Note that the impurity element concentration of the region 106 c is lower than the impurity element concentration of the region 106 b because the impurity element is added to the oxide semiconductor film 106 through the insulating film 108. After that, the mask 111 is removed as illustrated in FIG. 9A.

Note that although the impurity element 117 is added to the oxide semiconductor film 106 using the mask 111 here, the impurity element 117 may be added to the oxide semiconductor film 106 using the conductive films 110, 112, and 114, as masks after the mask 111 is removed.

After that, heat treatment may be performed to further increase the conductivity of the region to which the impurity element 117 is added. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, higher than or equal to 250° C. and lower than or equal to 450° C., or higher than or equal to 300° C. and lower than or equal to 450° C.

Next, as illustrated in FIG. 9B, the insulating film 116 is formed over the oxide semiconductor film 106, the insulating film 108, the conductive film 110, the conductive film 112, and the conductive film 114, and the insulating film 118 is formed over the insulating film 116.

The insulating film 116 and the insulating film 118 can be formed using the formation methods of the insulating film 104 and the insulating film 108 as appropriate.

Note that a silicon oxide film or a silicon oxynitride film from which oxygen can be released by heat treatment can be formed as the insulating film 116 under the following conditions: the substrate placed in a treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., or higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, or greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², or greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

Alternatively, heat treatment is performed after an aluminum film or an aluminum oxide film is formed over the oxide semiconductor film 106, the conductive film 110, the conductive film 112, the conductive film 114, whereby oxygen contained in the oxide semiconductor film 106 reacts with the aluminum film or the aluminum oxide film in the region 106 b illustrated in FIG. 2; thus, an aluminum oxide film is formed as the insulating film 116, and oxygen vacancies are formed in the region 106 b illustrated in FIG. 2. As a result, the conductivity of the region 106 b can be further increased.

After that, heat treatment may be performed to further increase the conductivity of the region to which the impurity element 117 is added. The temperature of the heat treatment is typically higher than or equal to 150° C. and lower than the strain point of the substrate, higher than or equal to 250° C. and lower than or equal to 450° C., or higher than or equal to 300° C. and lower than or equal to 450° C.

Through the above-described steps, the transistor can be manufactured.

<Method 2 for Manufacturing Semiconductor Device>

A method for manufacturing the transistor 151 illustrated in FIGS. 3A to 3C is described. Note that here, a step of forming the conductive film 110 c, the conductive film 112 c, and the conductive film 114 c that are included in the conductive film 110, the conductive film 112, and the conductive film 114 in the transistor 151 and a step of adding the impurity element 117 to the oxide semiconductor film 106 are described.

Through the steps illustrated in FIGS. 7A to 7D and FIGS. 8A to 8D, the insulating film 104, the oxide semiconductor film 106, the insulating film 108, the conductive film 110, the conductive film 112, the conductive film 114, and the mask 111 are formed over the substrate 102.

Next, as illustrated in FIG. 8D, the impurity element 117 is added to the oxide semiconductor film 106.

Then, the mask 111 is removed.

Next, the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b that are included in the conductive film 110, the conductive film 112, and the conductive film 114, respectively, are exposed to plasma generated in a reducing atmosphere, so that oxides on surfaces of the conductive film 110 b, the conductive film 112 b, and the conductive film 114 b are reduced. Then, the conductive films 110 b, 112 b, and 114 b are exposed to silane while heating is performed at a temperature higher than or equal to 200° C. and lower than or equal to 400° C. Next, the conductive films 110 b, 112 b, and 114 b are exposed to plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, whereby CuSi_(x)N_(y) (x>0, y>0) can be formed as the conductive film 110 c, the conductive film 112 c, and the conductive film 114 c.

Note that in performing the exposure to the plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, the oxide semiconductor film 106 is exposed to the plasma generated in the atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen. Therefore, nitrogen and/or hydrogen can be added to the oxide semiconductor film 106.

Note that before the impurity element 117 is added to the oxide semiconductor film 106, the mask 111 may be removed and the conductive film 110 c, the conductive film 112 c, and the conductive film 114 c which are included in the conductive film 110, the conductive film 112, and the conductive film 114 may be formed.

Then, the step illustrated in FIG. 9B is performed. In this manner, the transistor 151 can be manufactured.

In the transistor described in this embodiment, the conductive films 110 and 112 and the conductive film 114 do not overlap each other; thus, parasitic capacitance can be reduced, resulting in a large on-state current. Furthermore, in the transistor described in this embodiment, the low-resistance region can be formed stably; thus, the on-state current is higher than that of a conventional transistor and variations in the electrical characteristics are smaller than those in a conventional transistor.

Although the case where a channel or the like is formed in an oxide semiconductor film is described in this embodiment or the like, one embodiment of the present invention is not limited thereto. For example, depending on circumstances or conditions, a material containing Si (silicon), Ge (germanium), SiGe (silicon germanium), GaAs (gallium arsenide), or the like may be used for a channel, the vicinity of the channel, a source region, a drain region, and the like.

The structure and method described in this embodiment can be combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 2

In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device are described with reference to FIGS. 12A to 12C, FIG. 13, FIGS. 14A to 14C, FIGS. 15A to 15C, FIGS. 16A to 16C, FIGS. 17A to 17C, FIGS. 18A and 18B, FIGS. 19A to 19D, FIGS. 20A and 20B, FIGS. 21A to 21C, and FIGS. 22A to 22C. Note that the difference between this embodiment and Embodiment 1 is in a method for forming the low-resistance region.

<Structure 5 of Semiconductor Device>

In FIGS. 12A to 12C, a transistor having a top-gate structure is illustrated as an example of a transistor included in a semiconductor device.

FIGS. 12A to 12C are a top view and cross-sectional views of a transistor 190 included in a semiconductor device. FIG. 12A is a top view of the transistor 190, FIG. 12B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 12A, and FIG. 12C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 12A.

The transistor 190 illustrated in FIGS. 12A to 12C includes an oxide semiconductor film 166 over an insulating film 164 formed over a substrate 162, an insulating film 168 in contact with the oxide semiconductor film 166, a conductive film 170 in contact with the oxide semiconductor film 166 in part of an opening portion 180 a in the insulating film 168, a conductive film 172 in contact with the oxide semiconductor film 166 in part of an opening portion 180 b in the insulating film 168, and a conductive film 174 overlapping the oxide semiconductor film 166 with the insulating film 168 provided therebetween. Note that an insulating film 176 is provided over the transistor 190. Furthermore, an insulating film 178 may be provided over the insulating film 176.

In the oxide semiconductor film 166, elements that form oxygen vacancies are contained in a region that is not overlapped with the conductive film 170, the conductive film 172, and the conductive film 174. Hereinafter, the elements that form oxygen vacancies are described as impurity elements. Typical examples of the impurity elements are hydrogen, rare gas elements, and the like. Typical examples of the rare gas elements are helium, neon, argon, krypton, and xenon. Furthermore, as the impurity element, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, chlorine, or the like may be contained in the oxide semiconductor film 166.

The insulating film 176 is a film containing hydrogen, and a nitride insulating film is a typical example thereof. The insulating film 176 is in contact with the oxide semiconductor film 166. Thus, hydrogen contained in the insulating film 176 is diffused into the oxide semiconductor film 166. As a result, a large amount of hydrogen is contained in a region in contact with the insulating film 176 in the oxide semiconductor film 166.

When a rare gas element is added as an impurity element to the oxide semiconductor film, a bond between a metal element and oxygen in the oxide semiconductor film is cut, whereby an oxygen vacancy is formed. By interaction between hydrogen and the oxygen vacancy included in the oxide semiconductor film, the conductivity of the oxide semiconductor film is increased. Specifically, when hydrogen enters the oxygen vacancy in the oxide semiconductor film, an electron serving as a carrier is generated. As a result, the conductivity is increased.

FIG. 13 is an enlarged view of the oxide semiconductor film 166 and the vicinity thereof. As illustrated in FIG. 13, the oxide semiconductor film 166 includes a region 166 a in contact with the conductive film 170 or the conductive film 172, a region 166 b in contact with the insulating film 176, and a region 166 c and a region 166 d that are overlapped with the insulating film 168.

The regions 166 a serve as a source region and a drain region. Like the regions 106 a described in Embodiment 1, the regions 166 a in contact with the conductive film 170 and the conductive film 172 have high conductivity and serve as a source region and a drain region.

The region 166 b and the region 166 c serve as low-resistance regions. The region 166 b and the region 166 c contain at least a rare gas and hydrogen as the impurity elements. Note that the impurity element concentration of the region 166 b is higher than the impurity element concentration of the region 166 c. In the case where a side surface of the conductive film 174 has a tapered shape, part of the region 166 c may be overlapped with the conductive film 174.

In the case where the oxide semiconductor film 166 is formed by a sputtering method, the regions 166 a to 166 d each contain a rare gas element. In addition, the rare gas element concentration of each of the regions 166 b and 166 c is higher than that of each of the regions 166 a and 166 d. The reasons are as follows: in the case where the oxide semiconductor film 166 is formed by a sputtering method, a rare gas is used as a sputtering gas, so that the oxide semiconductor film 166 contains the rare gas; and a rare gas is intentionally added to the regions 166 b and 166 c in order to form oxygen vacancies in the regions 166 b and 166 c. Note that a rare gas element different from that added to the regions 166 a and 166 d may be added to the regions 166 b and 166 c.

Furthermore, in the case where the amount of oxygen vacancy is larger than the amount of hydrogen in the region 166 b and the region 166 c, the carrier density of the region 166 b and the region 166 c can be controlled by controlling the amount of hydrogen. Alternatively, in the case where the amount of hydrogen is larger than the amount of oxygen vacancy in the region 166 b and the region 166 c, the carrier density of the region 166 b and the region 166 c can be controlled by controlling the amount of oxygen vacancy. Note that when the carrier density of the region 166 b and the region 166 c is greater than or equal to 5×10¹⁸/cm³, preferably greater than or equal to 1×10¹⁹/cm³, further preferably greater than or equal to 1×10²⁰/cm³, a transistor in which resistance between the channel and the source and the drain regions is small and on-state current is high can be formed.

Since the region 166 b is in contact with the insulating film 176, the hydrogen concentration of the region 166 b is higher than those of the region 166 a and the region 166 d. In the case where hydrogen is diffused from the region 166 b to the region 166 c, the hydrogen concentration of the region 166 c is higher than those of the region 166 a and the region 166 d. Note that the hydrogen concentration of the region 166 b is higher than that of the region 166 c.

In the regions 166 b and 166 c, the hydrogen concentration, which is measured by SIMS, can be higher than or equal to 8×10¹⁹ atoms/cm³, higher than or equal to 1×10²⁰ atoms/cm³, or higher than or equal to 5×10²⁰ atoms/cm³. Note that in the regions 166 a and 166 d, the hydrogen concentration, which is measured by SIMS, can be lower than or equal to 5×10¹⁹ atoms/cm³, lower than or equal to 1×10¹⁹ atoms/cm³, lower than or equal to 5×10¹⁸ atoms/cm³, lower than or equal to 1×10¹⁸ atoms/cm³, lower than or equal to 5×10¹⁷ atoms/cm³, or lower than or equal to 1×10¹⁶ atoms/cm³.

In the case where boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, or chloride is added as the impurity element to the oxide semiconductor film 166, the impurity element is contained in only the regions 166 b and 166 c. Thus, the impurity element concentration of each of the regions 166 b and 166 c is higher than that of each of the regions 166 a and 166 d. Note that, in the region 166 b and the region 166 c, the impurity element concentration, which is measured by SIMS, can be higher than or equal to 5×10¹⁸ atoms/cm³ and lower than or equal to 1×10²² atoms/cm³, higher than or equal to 1×10¹⁹ atoms/cm³ and lower than or equal to 1×10²¹ atoms/cm³, or higher than or equal to 5×10¹⁹ atoms/cm³ and lower than or equal to 5×10²⁰ atoms/cm³.

The regions 166 b and 166 c have higher hydrogen concentrations than the region 166 d and have more oxygen vacancies than the region 166 d because of addition of impurity elements. Therefore, the regions 166 b and 166 c have higher conductivity and serve as low-resistance regions. Thus, the resistivity of the regions 166 b and 166 c can be typically greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁴ Ωcm, or greater than or equal to 1×10⁻³ Ωcm and less than 1×10⁻¹ Ωcm.

Note that in the region 166 b and the region 166 c, when the amount of hydrogen is smaller than or equal to the amount of oxygen vacancy, hydrogen is easily captured by the oxygen vacancy and is not easily diffused into the region 166 d that serves as a channel. As a result, a normally-off transistor can be manufactured.

The region 166 d serves as a channel.

In the insulating film 168, a region that overlaps the oxide semiconductor film 166 and is overlapped with the conductive film 174 serves as a gate insulating film. In addition, in the insulating film 168, regions that overlap the oxide semiconductor film 166 and are overlapped with the conductive films 170 and 172 serve as interlayer insulating films.

The conductive film 170 and the conductive film 172 serve as a source electrode and a drain electrode. The conductive film 174 serves as a gate electrode.

In the transistors 190 described in this embodiment, the regions 166 b and 166 c serving as low-resistance regions are provided between the region 166 d serving as a channel and the regions 166 a serving as a source region and a drain region. The resistance between the channel and the source region and the drain region can be reduced, and the transistor 190 and the transistor 194 have high on-state current and high field-effect mobility.

In a process of manufacturing the transistor 190, the conductive film 174 that serves as a gate electrode and the conductive films 170 and 172 that serve as a source electrode and a drain electrode are formed at the same time. Thus, in the transistor 190, the conductive film 174 does not overlap the conductive films 170 and 172, and the parasitic capacitance between the conductive film 174 and each of the conductive films 170 and 172 can be reduced. As a result, in the case where a large-area substrate is used as the substrate 162, signal delay in the conductive films 170, 172, and 174 can be reduced.

In the transistor 190, a region including oxygen vacancy is formed by adding the rare gas element to the oxide semiconductor film 166 using the conductive film 170, the conductive film 172, and the conductive film 174 as masks. Furthermore, because the region including oxygen vacancies is in contact with the insulating film 176 containing hydrogen, hydrogen contained in the insulating film 176 is diffused into the region including oxygen vacancies, so that a low-resistance region is formed. That is, the low-resistance region can be formed in a self-aligned manner.

In the transistor described in this embodiment, the rare gas is added to the region 166 b and the region 166 c to form oxygen vacancies, and furthermore, hydrogen is added. Thus, the conductivity of the region 166 b and the region 166 c can be increased, and variations in the conductivity of the region 166 b and the region 166 c between transistors can be reduced. That is, by adding the rare gas and hydrogen to the region 166 b and the region 166 c, the conductivity of the region 166 b and the region 166 c can be controlled.

The structure illustrated in FIGS. 12A to 12C is described in detail below.

As the substrate 162, the substrate 102 described in Embodiment 1 can be used as appropriate.

The insulating film 164 can be formed using any of the materials for the insulating film 104 given in Embodiment 1 as appropriate.

The oxide semiconductor film 166 can be formed using any of the materials and the structures of the oxide semiconductor film 106 given in Embodiment 1 as appropriate.

The insulating film 168 can be formed using any of the materials for the insulating film 118 given in Embodiment 1 as appropriate.

The conductive films 170, 172, and 174 can be formed using any of the materials for the conductive films 110, 112, and 114 given in Embodiment 1 as appropriate.

The insulating film 176 is a film containing hydrogen, and a nitride insulating film is a typical example thereof. The nitride insulating film can be formed using silicon nitride, aluminum nitride, or the like.

The insulating film 178 can be formed using any of the materials for the insulating film 118 given in Embodiment 1 as appropriate.

<Structure 6 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 14A to 14C.

FIGS. 14A to 14C are a top view and cross-sectional views of a transistor 191 included in a semiconductor device. FIG. 14A is a top view of the transistor 191, FIG. 14B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 14A, and FIG. 14C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 14A.

The transistor 191 illustrated in FIGS. 14A to 14C is characterized in that the conductive film 170, the conductive film 172, and the conductive film 174 each have a three-layer structure and in that the insulating film 164 has a stacked-layer structure of a nitride insulating film 164 a and an oxide insulating film 164 b. The other components are the same as those of the transistor 190, and the effect similar to that in the case of the transistor 190 is obtained.

First, the conductive films 170, 172, and 174 are described.

In the conductive film 170, a conductive film 170 a, a conductive film 170 b, and a conductive film 170 c are stacked in this order, and the conductive film 170 a and the conductive film 170 c cover surfaces of the conductive film 170 b. That is, the conductive film 170 a and the conductive film 170 c serve as protective films of the conductive film 170 b.

In the conductive film 172, a conductive film 172 a, a conductive film 172 b, and a conductive film 172 c are stacked in this order, and the conductive film 172 a and the conductive film 172 c cover surfaces of the conductive film 172 b, as in the case of the conductive film 170.

In the conductive film 174, a conductive film 174 a, a conductive film 174 b, and a conductive film 174 c are stacked in this order, and the conductive film 174 a and the conductive film 174 c cover surfaces of the conductive film 174 b, as in the case of the conductive film 170.

Like the conductive films 110 a, 112 a, and 114 a in Embodiment 1, the conductive films 170 a, 172 a, and 174 a can be formed using a material that prevents metal elements contained in the conductive films 170 b, 172 b, and 174 b from being diffused into the oxide semiconductor film 166, as appropriate.

Like the conductive films 110 b, 112 b, and 114 b in Embodiment 1, the conductive films 170 b, 172 b, and 174 b can be formed using a low-resistance material as appropriate.

Like the conductive films 110 c, 112 c, and 114 c in Embodiment 1, the conductive films 170 c, 172 c, and 174 c can be formed using films in which metal elements contained in the conductive film 170 b, the conductive film 172 b, and the conductive film 174 b are passivated. As a result, the metal elements contained in the conductive films 170 b, 172 b, and 174 b can be prevented from moving to the oxide semiconductor film 166 in the step of forming the insulating film 176.

Next, the insulating film 164 in which the nitride insulating film 164 a and the oxide insulating film 164 b are stacked is described.

The nitride insulating film 164 a and the nitride insulating film 164 b can be formed using any of the materials for the nitride insulating film 104 a and the oxide insulating film 104 b given in Embodiment 1, as appropriate.

<Structure 7 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 15A to 15C and FIGS. 16A to 16C.

FIGS. 15A to 15C are a top view and cross-sectional views of a transistor 192 included in a semiconductor device. FIG. 15A is a top view of the transistor 192, FIG. 15B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 15A, and FIG. 15C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 15A.

The transistor 192 illustrated in FIGS. 15A to 15C is characterized in that the oxide semiconductor film 166 has a multilayer structure. Specifically, the oxide semiconductor film 166 includes an oxide semiconductor film 167 a in contact with the insulating film 164, an oxide semiconductor film 167 b in contact with the oxide semiconductor film 167 a, and an oxide semiconductor film 167 c in contact with the oxide semiconductor film 167 b, the conductive film 170, the conductive film 172, the insulating film 168, and the insulating film 176. The other components are the same as those of the transistor 190 and the effect similar to that in the case of the transistor 190 is obtained.

The oxide semiconductor films 167 a, 167 b, and 167 c can be formed using any of any of the materials and the crystalline structure for the oxide semiconductor films 107 a, 107 b, and 107 c given in Embodiment 1, as appropriate.

The oxide semiconductor film 167 a and the oxide semiconductor film 167 c in each of which oxygen vacancies are less likely to be generated than in the oxide semiconductor film 167 b are provided in contact with an upper surface and a lower surface of the oxide semiconductor film 167 b, whereby oxygen vacancies in the oxide semiconductor film 167 b can be reduced. Furthermore, because the oxide semiconductor film 167 b is in contact with the oxide semiconductor film 167 a and the oxide semiconductor film 167 c that contain one or more metal elements forming the oxide semiconductor film 167 b, the densities of interface levels at the interface between the oxide semiconductor film 167 a and the oxide semiconductor film 167 b and at the interface between the oxide semiconductor film 167 b and the oxide semiconductor film 167 c are extremely low. Accordingly, oxygen vacancies in the oxide semiconductor film 167 b can be reduced.

Providing the oxide semiconductor film 167 a makes it possible to reduce variations in the electrical characteristics of the transistor, such as a threshold voltage.

Since the oxide semiconductor film 167 c containing one or more kinds of metal elements forming the oxide semiconductor film 167 b is provided in contact with the oxide semiconductor film 167 b, scattering of carriers does not easily occur at the interface between the oxide semiconductor film 167 b and the oxide semiconductor film 167 c, and thus the field-effect mobility of the transistor can be increased.

The oxide semiconductor film 167 a and the oxide semiconductor film 167 c each also serve as a barrier film which suppresses formation of an impurity state due to the entry of the constituent element of the insulating films 164 and 168 or the constituent element of the conductive films 170 and 172 into the oxide semiconductor film 167 b.

From the above, variations in the electrical characteristics such as threshold voltage are reduced in the transistor described in this embodiment.

FIGS. 16A to 16C illustrate a transistor having a structure different from that illustrated in FIGS. 15A to 15C.

FIGS. 16A to 16C are a top view and cross-sectional views of a transistor 193 included in a semiconductor device. FIG. 16A is a top view of the transistor 193, FIG. 16B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 16A, and FIG. 16C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 16A.

As in the transistor 193 illustrated in FIGS. 16A to 16C, the oxide semiconductor film 166 may have a stacked-layer structure including the oxide semiconductor film 167 b in contact with the insulating film 164 and the oxide semiconductor film 167 c in contact with the oxide semiconductor film 167 b and the insulating film 168. The other components are the same as those of the transistor 190 and the effect similar to that in the case of the transistor 190 is obtained.

<Structure 8 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 17A to 17C.

FIGS. 17A to 17C are a top view and cross-sectional views of a transistor 194 included in a semiconductor device. FIG. 17A is a top view of the transistor 194, FIG. 17B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 17A, and FIG. 17C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 17A.

A transistor 194 illustrated in FIGS. 17A to 17C is characterized in that a conductive film 181 is provided so as to be overlapped with the oxide semiconductor film 166 with the insulating film 164 therebetween. That is, the conductive film 181 serves as a gate electrode. Furthermore, the transistor 194 is a transistor having a dual-gate structure.

The threshold voltage of the transistor 194 can be controlled by supplying different potentials to the conductive film 174 and the conductive film 181 that are not connected to each other. Alternatively, by supplying the same potential to the conductive film 174 and the conductive film 181 that are connected to each other through an opening 183 as illustrated in FIG. 17B, variations in the initial characteristics can be reduced, and degradation of the transistor due to a −GBT stress test and a change in the rising voltage of the on-state current at different drain voltages can be suppressed. In addition, a region where carriers flow through the oxide semiconductor film 166 becomes large in the film thickness direction, so that the amount of carrier movement is increased. As a result, the on-state current and field-effect mobility of the transistor 194 are increased. When the channel length of the transistor is less than 2.5 μm, preferably greater than or equal to 1.45 μm and less than or equal to 2.2 μm, the on-state current can be further increased and the field-effect mobility can be increased.

<Structure 9 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 25A to 25C and FIGS. 26A to 26C.

FIGS. 25A to 25C are a top view and cross-sectional views of a transistor 150A included in a semiconductor device. FIG. 25A is a top view of the transistor 150A, FIG. 25B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 25A, and FIG. 25C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 25A. FIGS. 26A to 26C are a top view and cross-sectional views of a transistor 190A included in a semiconductor device. FIG. 26A is a top view of the transistor 190A, FIG. 26B is a cross-sectional view taken along the dashed-dotted line Y1-Y2 in FIG. 26A, and FIG. 26C is a cross-sectional view taken along the dashed-dotted line X1-X2 in FIG. 26A.

The transistor 150A illustrated in FIGS. 25A to 25C is a modification example of the transistor 150 illustrated in FIGS. 1A to 1C. In the transistor 150A, the insulating film 108 has a different shape from the insulating film 108 in the transistor 150. In the transistor 150A illustrated in FIGS. 25A to 25C, the insulating film 108 is patterned into an island shape. The other components are the same as those of the transistor 150 and the effect similar to that in the case of the transistor 150 can be obtained.

The transistor 190A illustrated in FIGS. 26A to 26C is a modification example of the transistor 190 illustrated in FIGS. 12A to 12C and is different from the transistor 190 in the shape of the insulating film 168. In the transistor 190A illustrated in FIGS. 26A to 26C, the insulating film 168 is patterned into an island shape. The other components are the same as those of the transistor 190 and the effect similar to that in the case of the transistor 190 can be obtained.

By patterning the insulating film 108 into an island shape, the area where the oxide semiconductor film 106 is in contact with the conductive films 110 and 112 can be increased. This can reduce the contact resistance between the oxide semiconductor film 106 and the conductive films 110 and 112. In addition, by patterning the insulating film 168 into an island shape, the area where the oxide semiconductor film 166 is in contact with the conductive films 170 and 172 can be increased. This can reduce the contact resistance between the oxide semiconductor film 166 and the conductive films 170 and 172. Note that in the case where the insulating film 108 or the insulating film 168 is not patterned into an island shape, as in the transistor 150 illustrated in FIGS. 1A to 1C or the transistor 190 illustrated in FIGS. 12A to 12C, a structure in which an outer edge of the oxide semiconductor film 106 or the oxide semiconductor film 166 is covered with the insulating film 108 or the insulating film 168 can be employed. The above-described structure is preferably employed, in which case impurities can be prevented from entering the oxide semiconductor film 106 or 166. In the case where the insulating film 108 is patterned into an island shape, at least parts of the conductive film 114 serving as the gate electrode, the conductive film 110 serving as the source electrode, and the conductive film 112 serving as the drain electrode are formed on the same plane. In the case where the insulating film 168 is not patterned into an island shape, at least parts of the conductive film 174 serving as the gate electrode, the conductive film 170 serving as the source electrode, and the conductive film 172 serving as the drain electrode are formed on the same plane.

<Structure 10 of Semiconductor Device>

Next, another structure of a semiconductor device is described with reference to FIGS. 38A and 38B.

FIG. 38A is a cross-sectional view of a transistor 190B of the semiconductor device. FIG. 38B is a conceptual diagram in a thickness direction in the case where the impurity element is added to the oxide semiconductor film 166. Note that a top view and a cross-sectional view in the channel width direction of the transistor 190B illustrated in FIG. 38A are similar to the top view in FIG. 12B and the cross-sectional view in FIG. 12A, respectively; thus, they are not described here.

The transistor 190B illustrated in FIG. 38A is a modification example of the transistor 190 illustrated in FIGS. 12A to 12C. In the transistor 190B, the conductive films 170, 172, and 174 are different from those in the transistor 190. In addition, the cross-sectional shapes of the insulating films 168, 176, and 178 are different from those of the insulating films 168, 176, and 178 in the transistor 190. In the transistor 190B illustrated in FIG. 38A, the conductive films 170, 172, and 174 each have a two-layer structure, and parts of end portions of the insulating films 168, 176, and 178 have curvature. The other components are the same as those of the transistor 190 and the effect similar to that in the case of the transistor 190 can be obtained.

The conductive film 170 has a stacked-layer structure of a conductive film 170 d and a conductive film 170 e. The conductive film 172 has a stacked-layer structure of a conductive film 172 d and a conductive film 172 e. The conductive film 174 has a stacked-layer structure of a conductive film 174 d and a conductive film 174 e. As the conductive films 170 d, 172 d, and 174 d, for example, a film of metal nitride such as tantalum nitride, titanium nitride, molybdenum nitride, or tungsten nitride can be used.

The conductive films 170 e, 172 e, and 174 e can be formed using a low-resistance metal material. Examples of the low-resistance metal material include aluminum, copper, and silver. Other than the low-resistance metal materials given above, tungsten or molybdenum can be used for the conductive films 170 e, 172 e, and 174 e.

In the conductive film 170, an end portion of the conductive film 170 d extends beyond an end portion of the conductive film 170 e. In the conductive film 172, an end portion of the conductive film 172 d extends beyond an end portion of the conductive film 172 e. In the conductive film 174, an end portion of the conductive film 174 d extends beyond an end portion of the conductive film 174 e. As described above, the conductive films 170, 172, and 174 can each have the two-layer structure in which the lower conductive film extends beyond the end portion of the upper conductive film. The lower conductive film extending beyond the end portion of the upper conductive film can prevent passing of impurities at the time of addition of the impurities in some cases.

Examples of a method for processing the conductive films 170, 172, and 174 include a dry etching method. When the conductive films 170, 172, and 174 are processed by the dry etching method, part of an end portion of the insulating film 168 is reduced, so that the end portion has curvature in some cases. In the case where the end portion of the insulating film 168 has curvature, parts of end portions of the insulating films 176 and 178 formed over the insulating film 168 also have curvature in some cases because of the effect of the insulating film 168.

A conceptual diagram in the thickness direction in the case where an impurity element is added to the oxide semiconductor film 166 of the transistor 190B illustrated in FIG. 38A is described below with reference to FIG. 38B.

In FIG. 38B, the oxide semiconductor film 166 includes a region 166 x and a region 166 y. In the case where the oxide semiconductor film 166 is a crystalline oxide semiconductor film, for example, the crystallinity of the region 166 y is higher than that of the region 166 x. The difference in crystallinity is due to a decrease in the crystallinity of the region 166 x that is damaged when the impurity element is added.

<Method 3 for Manufacturing Semiconductor Device>

Next, a method for manufacturing the transistor 190 illustrated in FIGS. 12A to 12C is described with reference to FIGS. 18A and 18B, FIGS. 19A to 19D, and FIGS. 20A and 20B.

As illustrated in FIG. 18A, the insulating film 164 is formed over the substrate 162.

The insulating film 164 can be formed by the formation method of the insulating film 104 in Embodiment 1 as appropriate.

Next, the oxide semiconductor film 166 is formed over the insulating film 164 as illustrated in FIG. 18B. Then, the insulating film 168 is formed over the insulating film 164 and the oxide semiconductor film 166. The oxide semiconductor film 166 and the insulating film 168 can be formed by the formation method of the oxide semiconductor film 106 and the insulating film 108 in Embodiment 1, as appropriate.

Next, after a mask is formed over the insulating film 168 by lithography, the insulating film 168 is partly etched to form the opening portion 180 a and the opening portion 180 b through which parts of the oxide semiconductor film 166 are exposed as illustrated in FIG. 19A.

Then, as illustrated in FIG. 19B, the conductive film 169 is formed over the oxide semiconductor film 166 and the insulating film 168.

The conductive film 169 can be formed by the formation method of the conductive film 109 in Embodiment 1 as appropriate.

Next, after a mask 111 is formed over the conductive film 169 by lithography, the conductive film 169 is exposed to an etchant and/or an etching gas 167, whereby the conductive film 170, the conductive film 172, and the conductive film 174 are formed as illustrated in FIG. 19C.

As a method for etching the conductive film 169, a wet etching method and/or a dry etching method can be employed as appropriate.

Note that the conductive films 170, 172, and 174 may be formed by an electrolytic plating method, a printing method, an inkjet method, or the like instead of the formation method.

Next, as illustrated in FIG. 19D, a rare gas is added as the impurity element 177 to the oxide semiconductor film 166 and the oxide semiconductor film 226 with the mask 111 left. As a result, the impurity element is added to regions that are not covered with the mask 111 in the oxide semiconductor films. Note that by the addition of the impurity element 177, oxygen vacancies are formed in the oxide semiconductor film.

As a method for adding the impurity element 177, the method for adding the impurity element 117 described in Embodiment 1 can be used as appropriate.

FIGS. 21A to 21C are conceptual diagrams of a region to which the impurity element 177 is added in a thickness direction when the impurity element is added to the oxide semiconductor film 166. FIGS. 21A to 21C are enlarged views of the oxide semiconductor film 166 and the vicinity thereof.

As illustrated in FIG. 21A, a region to which the impurity element 177 is added is formed in the insulating film 164, the oxide semiconductor film 166, and the insulating film 168 in some cases. Note that an end portion 195 of the region to which the impurity element 177 is added is positioned inside the insulating film 164 in a depth direction of a region where the oxide semiconductor film 166 is exposed.

As illustrated in FIG. 21B, a region to which the impurity element 177 is added is formed in the oxide semiconductor film 166 and the insulating film 168 in some cases. Note that an end portion 196 of the region to which the impurity element 177 is added is positioned at the interface between the insulating film 164 and the oxide semiconductor film 166 in a depth direction of a region where the oxide semiconductor film 166 is exposed.

As illustrated in FIG. 21C, a region to which the impurity element 177 is added is formed in the oxide semiconductor film 166 and the insulating film 168 in some cases. Note that an end portion 197 of the region to which the impurity element 177 is added is positioned inside the oxide semiconductor film 166 in a depth direction of a region where the oxide semiconductor film 166 is exposed.

After that, the mask 111 is removed as illustrated in FIG. 20B.

Note that here, the impurity element 177 is added to the oxide semiconductor film 166 using the mask 111; alternatively, the impurity element 177 may be added to the oxide semiconductor film 166 using the conductive films 170, 172, and 174 as masks after the mask 111 is removed.

In the case where the oxide semiconductor film 166 is damaged and oxygen vacancies are formed in a step of forming the conductive film 169, a step of etching the conductive film 169, or a step of forming the insulating film 176 which is to be described later, the impurity element 177 is not necessarily added.

Next, as illustrated in FIG. 20B, the insulating film 176 may be formed over the oxide semiconductor film 166, the insulating film 168, and the conductive films 170, 172, and 174, and the insulating film 178 may be formed over the insulating film 176.

As a method for forming the insulating film 176, a sputtering method, a CVD method, a vacuum evaporation method, a pulsed laser deposition (PLD) method, or the like is given. Note that a silicon nitride film containing hydrogen can be formed by a plasma CVD method using silane and ammonia as a source gas or using silane and nitrogen as a source gas. Furthermore, by using a plasma CVD method, the oxide semiconductor film 166 can be damaged, and oxygen vacancy can be formed in the oxide semiconductor film 166.

Since hydrogen is contained in the insulating film 176, when the insulating film 176 is in contact with the region to which the impurity element is added in the oxide semiconductor film 166, hydrogen contained in the insulating film 176 moves to the region to which the impurity element is added in the oxide semiconductor film. Since oxygen vacancy is included in the region to which the impurity element is added, the low-resistance region can be formed in the oxide semiconductor film 166. Specifically, the region 166 b and the region 166 c illustrated in FIG. 13 can be formed. Note that the impurity element concentration of the region 166 c is lower than the impurity element concentration of the region 166 b because the impurity element is added to the oxide semiconductor film 166 through the insulating film 168.

By forming the insulating film 176 while heating is performed, hydrogen contained in the oxide semiconductor film is diffused. However, when hydrogen moves to the oxygen vacancy, the hydrogen becomes stable in terms of energy; therefore, hydrogen is unlikely to move from the oxygen vacancy. Furthermore, by interaction between the oxygen vacancy and the hydrogen, an electron serving as a carrier is produced. Thus, by forming the insulating film 176 while heating is performed, the low-resistance region with small variation in conductivity can be formed.

After that, heat treatment may be performed to further increase the conductivity of the region to which the impurity element 177 is added. The heat treatment is performed typically at a temperature higher than or equal to 150° C. and lower than the strain point of the substrate, higher than or equal to 250° C. and lower than or equal to 450° C., or higher than or equal to 300° C. and lower than or equal to 450° C. As a result, the conductivity of the low-resistance region can be increased, and variation in the conductivity of the low-resistance region can be reduced

The insulating film 178 can be formed by any of the formation methods of the insulating films 164 and 168 as appropriate.

Note that, as the insulating film 178, a silicon oxide film or a silicon oxynitride film from which oxygen can be released by heat treatment can be formed under the following conditions: the substrate placed in a treatment chamber of the plasma CVD apparatus that is vacuum-evacuated is held at a temperature higher than or equal to 180° C. and lower than or equal to 280° C., or higher than or equal to 200° C. and lower than or equal to 240° C., the pressure is greater than or equal to 100 Pa and less than or equal to 250 Pa, or greater than or equal to 100 Pa and less than or equal to 200 Pa with introduction of a source gas into the treatment chamber, and a high-frequency power of greater than or equal to 0.17 W/cm² and less than or equal to 0.5 W/cm², or greater than or equal to 0.25 W/cm² and less than or equal to 0.35 W/cm² is supplied to an electrode provided in the treatment chamber.

Through the above-described steps, the transistor can be manufactured.

<Method 4 for Manufacturing Semiconductor Device>

A method for manufacturing the transistor 191 illustrated in FIGS. 14A to 14C is described. Note that here, a step of forming the conductive film 170 c, the conductive film 172 c, and the conductive film 174 c which are included in the conductive film 170, the conductive film 172, and the conductive film 174 in the transistor 191 and a step of adding the impurity element 177 to the oxide semiconductor film 166 are described.

Through the steps illustrated in FIGS. 18A and 18B and FIGS. 19A to 19C, the insulating film 164, the oxide semiconductor film 166, the insulating film 168, the conductive film 170, the conductive film 172, the conductive film 174, and the mask 111 are formed over the substrate 162.

Next, as illustrated in FIG. 19D, the impurity element 177 is added to the oxide semiconductor film 166.

Then, the mask 111 is removed.

Next, the conductive film 170 b, the conductive film 172 b, and the conductive film 174 b which are included in the conductive film 170, the conductive film 172, and the conductive film 174, respectively, are exposed to plasma generated in a reducing atmosphere so that oxides on surfaces of the conductive film 170 b, the conductive film 172 b, and the conductive film 174 b are reduced. Next, the conductive film 170 b, the conductive film 172 b, and the conductive film 174 b are exposed to silane while heating is performed at a temperature higher than or equal to 200° C. and lower than or equal to 400° C. Next, the conductive film 170 b, the conductive film 172 b, and the conductive film 174 b are exposed to plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, whereby CuSi_(x)N_(y) (x>0, y>0) can be formed as the conductive film 170 c, the conductive film 172 c, and the conductive film 174 c.

Note that in performing the exposure to the plasma generated in an atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen, the oxide semiconductor film 166 is exposed to the plasma generated in the atmosphere containing nitrogen, such as an atmosphere of ammonia or nitrogen. Therefore, nitrogen and/or hydrogen can be added to the oxide semiconductor film 166.

Note that before the impurity element 177 is added to the oxide semiconductor film 166, the mask 111 may be removed and the conductive film 170 c, the conductive film 172 c, and the conductive film 174 c which are included in the conductive film 170, the conductive film 172, and the conductive film 174 may be formed.

Then, the step illustrated in FIG. 20B is performed. In this manner, the transistor 191 can be manufactured.

<Method 5 for Manufacturing Semiconductor Device>

Another method for manufacturing the transistor 190 illustrated in FIGS. 12A to 12C is described. Note that a step of adding the impurity element and a step of forming the insulating film 176 are described here with reference to FIGS. 22A to 22C.

Through the steps illustrated in FIGS. 18A and 18B and FIGS. 19A to 19D, the insulating film 164, the oxide semiconductor film 166, the insulating film 168, the conductive film 170, the conductive film 172, the conductive film 174, and the mask 111 are formed over the substrate 162. After that, the mask 111 is removed as illustrated in FIG. 22A.

Next, as illustrated in FIG. 22B, after the insulating film 176 is formed over the oxide semiconductor film 166, the insulating film 168, the conductive film 170, the conductive film 172, and the conductive film 174, the impurity element 177 is added to the oxide semiconductor film 166 through the insulating film 176 using the conductive film 170, the conductive film 172, and the conductive film 174 as masks.

Then, the insulating film 178 may be formed as illustrated in FIG. 22C. Through the above-described process, the transistor 190 can be manufactured.

In the transistor described in this embodiment, the conductive films 170 and 172 and the conductive film 174 do not overlap each other; thus, parasitic capacitance can be reduced, resulting in a large on-state current. Furthermore, in the transistor described in this embodiment, the low-resistance region can be formed stably; thus, the on-state current is higher than that of a conventional transistor and variations in the electrical characteristics are more reduced than those in a conventional transistor.

The structure and method described in this embodiment can be combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 3

In this embodiment, one embodiment of a semiconductor device and one embodiment of a method for manufacturing the semiconductor device are described with reference to FIGS. 23A and 23B and FIGS. 24A and 24B.

<Structure 11 of Semiconductor Device>

FIGS. 23A and 23B illustrate structures of the transistor 150 and a capacitor 159 connected to the transistor 150, which are included in a semiconductor device.

FIGS. 23A and 23B are a top view and a cross-sectional view of the transistor 150 and the capacitor 159 included in the semiconductor device. FIG. 23A is a top view of the transistor 150 and the capacitor 159. FIG. 23B shows cross sections taken along the dashed-dotted lines X1-X2 and X3-X4 in FIG. 23A. Note that in FIG. 23A, the substrate 102, the insulating films 104, 108, 116, 118, and 122, and the like are omitted for simplicity.

The transistor 150 illustrated in FIGS. 23A and 23B has a structure similar to that of the transistor 150 in Embodiment 1.

The capacitor 159 includes an oxide semiconductor film 156 over the insulating film 104, the insulating film 118 in contact with the oxide semiconductor film 156, and a conductive film 124 over the insulating film 118.

An insulating film 122 is formed over the insulating film 118. The conductive film 124 is in contact with the conductive film 112 in an opening portion 142 a in the insulating films 116, 118, and 122. The conductive film 124 is in contact with the insulating film 118 in an opening portion 142 b in the insulating films 108, 116, 118, and 122.

As the insulating film 122, for example, an organic resin film of polyimide, acrylic, polyamide, or epoxy can be used. The insulating film 122 preferably has a thickness greater than or equal to 500 nm and less than or equal to 10 μm.

The conductive film 124 can be formed using a light-transmitting conductive material such as indium tin oxide, indium oxide including tungsten oxide, indium zinc oxide including tungsten oxide, indium oxide including titanium oxide, indium tin oxide including titanium oxide, indium zinc oxide, or indium tin oxide including silicon oxide.

The conductive film 124 can be fainted using a metal element reflecting light, such as silver, aluminum, chromium, copper, tantalum, titanium, molybdenum, or tungsten. Furthermore, a film formed using a metal element relecting light and a film formed using the above light-transmitting conductive material may be stacked.

The oxide semiconductor film 156 has a light-transmitting property because of being formed concurrently with the oxide semiconductor film 106. Furthermore, the impurity element is added to the oxide semiconductor film 156 in a manner similar to that of the region 106 b included in the oxide semiconductor film 106. Thus, the oxide semiconductor film 156 has conductivity.

In the case where the conductive film 124 is formed using a light-transmitting conductive material, the capacitor 159 has a light-transmitting property. Thus, by providing the capacitor 159 in the pixel of the display device, the aperture ratio in the pixel can be increased.

<Structure 12 of Semiconductor Device>

FIGS. 24A and 24B illustrate structures of the transistor 190 and a capacitor 199 connected to the transistor 150, which are included in a semiconductor device.

FIGS. 24A and 24B are a top view and a cross-sectional view of the transistor 190 and the capacitor 199 included in the semiconductor device. FIG. 24A is a top view of the transistor 190 and the capacitor 199. FIG. 24B is cross-sectional views taken along the dashed-dotted lines X1-X2 and X3-X4 in FIG. 24A. Note that in FIG. 24A, the substrate 162, the insulating films 164, 168, 176, 178, and 182, and the like are omitted for simplicity.

The transistor 190 illustrated in FIGS. 38A and 38B has a structure similar to that of the transistor 190 in Embodiment 2.

The capacitor 199 includes an oxide semiconductor film 198 over the insulating film 164, the insulating film 176 in contact with the oxide semiconductor film 198, and a conductive film 184 over the insulating film 176

An insulating film 182 is forming over the insulating film 178. The conductive film 184 is in contact with the conductive film 172 in an opening portion 182 a in the insulating film 176, the insulating film 178, and the insulating film 182. The conductive film 184 is in contact with the insulating film 176 in an opening portion 182 b in the insulating film 168, the insulating film 176, the insulating film 178, and the insulating film 182.

The insulating film 182 can be formed using the material for the insulating film 122 illustrated in FIG. 23B as appropriate.

The conductive film 184 can be formed using the material for the conductive film 124 illustrated in FIG. 23B as appropriate.

The oxide semiconductor film 198 has a light-transmitting property because of being formed through the same process as the oxide semiconductor film 166. In addition, the impurity element is added to the oxide semiconductor film 198 in a manner similar to that of the region 166 b included in the oxide semiconductor film 166. Thus, the oxide semiconductor film 198 has conductivity.

In the case where the conductive film 184 is formed using a light-transmitting conductive material, the capacitor 199 has a light-transmitting property. Thus, by providing the capacitor 199 in the pixel of the display device, the aperture ratio in the pixel can be increased.

As one electrode of the capacitor, the oxide semiconductor film having conductivity can be formed through the same process as the oxide semiconductor film included in the transistor. Thus, the transistor and the capacitor can be formed at the same time without an increase in the number of masks.

The structure and method described in this embodiment can be combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 4

In this embodiment, the structure of an oxide semiconductor film included in a semiconductor device of one embodiment of the present invention is described below in detail.

First, structures of the oxide semiconductor film are described below.

<Structure of Oxide Semiconductor>

An oxide semiconductor is classified into a single crystal oxide semiconductor and a non-single-crystal oxide semiconductor. Examples of a non-single-crystal oxide semiconductor include CAAC-OS, a polycrystalline oxide semiconductor, a microcrystalline oxide semiconductor, and an amorphous oxide semiconductor.

From another perspective, an oxide semiconductor is classified into an amorphous oxide semiconductor and a crystalline oxide semiconductor. Examples of a crystalline oxide semiconductor include a single crystal oxide semiconductor, a CAAC-OS, a polycrystalline oxide semiconductor, and a microcrystalline oxide semiconductor.

<CAAC-OS>

First, a CAAC-OS is described. Note that a CAAC-OS can be referred to as an oxide semiconductor including c-axis aligned nanocrystals (CANC).

A CAAC-OS film is an oxide semiconductor film having a plurality of c-axis aligned crystal parts (also referred to as pellets).

In a combined analysis image (also referred to as a high-resolution TEM image) of a bright-field image and a diffraction pattern of a CAAC-OS, which is obtained using a transmission electron microscope (TEM), a plurality of pellets can be observed. However, in the high-resolution TEM image, a boundary between pellets, that is, a grain boundary is not clearly observed. Thus, in the CAAC-OS, a reduction in electron mobility due to the grain boundary is less likely to occur.

A CAAC-OS observed with TEM is described below. FIG. 34A shows a high-resolution TEM image of a cross section of the CAAC-OS observed from a direction substantially parallel to the sample surface. The high-resolution TEM image is obtained with a spherical aberration corrector function. The high-resolution TEM image obtained with a spherical aberration corrector function is particularly referred to as a Cs-corrected high-resolution TEM image. The Cs-corrected high-resolution TEM image can be obtained with, for example, an atomic resolution analytical electron microscope JEM-ARM200F manufactured by JEOL Ltd.

FIG. 34B is an enlarged Cs-corrected high-resolution TEM image of a region (1) in FIG. 34A. FIG. 34B shows that metal atoms are arranged in a layered manner in a pellet. Each metal atom layer has a configuration reflecting unevenness of a surface over which the CAAC-OS is formed (hereinafter, the surface is referred to as a formation surface) or a top surface of the CAAC-OS, and is arranged parallel to the formation surface or the top surface of the CAAC-OS.

As shown in FIG. 34B, the CAAC-OS layer has a characteristic atomic arrangement. The characteristic atomic arrangement is denoted by an auxiliary line in FIG. 34C. FIGS. 34B and 34C prove that the size of a pellet is approximately 1 nm to 3 nm, and the size of a space caused by tilt of the pellets is approximately 0.8 nm. Therefore, such a pellet can also be referred to as a nanocrystal (nc).

Here, according to the Cs-corrected high-resolution TEM images, the schematic arrangement of pellets 5100 of a CAAC-OS over a substrate 5120 is illustrated by such a structure in which bricks or blocks are stacked (see FIG. 34D). The part in which the pellets are tilted as observed in FIG. 34C corresponds to a region 5161 shown in FIG. 34D.

FIG. 35A shows a Cs-corrected high-resolution TEM image of a plane of the CAAC-OS layer observed from a direction substantially perpendicular to the sample surface. FIGS. 35B, 35C, and 35D are enlarged Cs-corrected high-resolution TEM images of regions (1), (2), and (3) in FIG. 35A, respectively. FIGS. 35B, 35C, and 35D indicate that metal atoms are arranged in a triangular, quadrangular, or hexagonal configuration in a pellet. However, there is no regularity of arrangement of metal atoms between different pellets.

Next, a CAAC-OS analyzed by X-ray diffraction (XRD) is described. For example, when the structure of a CAAC-OS including an InGaZnO₄ crystal is analyzed by an out-of-plane method, a peak appears at a diffraction angle (2θ) of around 31° as shown in FIG. 36A. This peak is derived from the (009) plane of the InGaZnO₄ crystal, which indicates that crystals in the CAAC-OS have c-axis alignment, and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS.

Note that in structural analysis of the CAAC-OS by an out-of-plane method, another peak may appear when 2θ is around 36°, in addition to the peak at 2θ of around 31°. The peak of 2θ at around 36° indicates that a crystal having no c-axis alignment is included in part of the CAAC-OS. It is preferable that in the CAAC-OS analyzed by an out-of-plane method, a peak appear when 2θ is around 31° and that a peak not appear when 2θ is around 36°.

On the other hand, in structural analysis of the CAAC-OS by an in-plane method in which an X-ray is incident on a sample in a direction substantially perpendicular to the c-axis, a peak appears when 2θ is around 56°. This peak is derived from the (110) plane of the InGaZnO₄ crystal. In the case of the CAAC-OS film, when analysis (φ scan) is performed with 2θ fixed at around 56° and with the sample rotated using a normal vector of the sample surface as an axis (φ axis), as shown in FIG. 36B, a peak is not clearly observed. In contrast, in the case of a single crystal oxide semiconductor of InGaZnO₄, when φ scan is performed with 2θ fixed at around 56°, as shown in FIG. 36C, six peaks which are derived from crystal planes equivalent to the (110) plane are observed. Accordingly, the structural analysis using XRD shows that the directions of a-axes and b-axes are different in the CAAC-OS.

Next, a CAAC-OS analyzed by electron diffraction is described. For example, when an electron beam with a probe diameter of 300 nm is incident on a CAAC-OS including an InGaZnO₄ crystal in a direction parallel to the sample surface, a diffraction pattern (also referred to as a selected-area transmission electron diffraction pattern) shown in FIG. 42A might be obtained. In this diffraction pattern, spots derived from the (009) plane of an InGaZnO₄ crystal are included. Thus, the electron diffraction also indicates that pellets included in the CAAC-OS have c-axis alignment and that the c-axes are aligned in a direction substantially perpendicular to the formation surface or the top surface of the CAAC-OS. Meanwhile, FIG. 42B shows a diffraction pattern obtained in such a manner that an electron beam with a probe diameter of 300 nm is incident on the same sample in a direction perpendicular to the sample surface. As shown in FIG. 42B, a ring-like diffraction pattern is observed. Thus, the electron diffraction also indicates that the a-axes and b-axes of the pellets included in the CAAC-OS do not have regular alignment. The first ring in FIG. 42B is considered to be derived from the (010) plane, the (100) plane, and the like of the InGaZnO₄ crystal. The second ring in FIG. 42B is considered to be derived from the (110) plane and the like.

Moreover, the CAAC-OS is an oxide semiconductor having a low density of defect states. Defects in the oxide semiconductor are, for example, defects due to impurities and oxygen vacancies. Therefore, the CAAC-OS can be regarded as an oxide semiconductor with a low impurity concentration, or an oxide semiconductor having a small number of oxygen vacancies.

Impurities contained in the oxide semiconductor might serve as carrier traps or carrier generation sources. Oxygen vacancies in the oxide semiconductor serve as carrier traps or serve as carrier generation sources when hydrogen is captured therein.

Note that the impurity means an element other than the main components of the oxide semiconductor, such as hydrogen, carbon, silicon, or a transition metal element. For example, an element (specifically, silicon) having higher strength of bonding to oxygen than a metal element included in an oxide semiconductor extracts oxygen from the oxide semiconductor, which results in disorder of the atomic arrangement and reduced crystallinity of the oxide semiconductor. A heavy metal such as iron and nickel, argon, and carbon dioxide, for example, has a large atomic radius (or molecular radius), and thus disturbs the atomic arrangement of the oxide semiconductor and decreases crystallinity.

An oxide semiconductor having a low density of defect states (a small number of oxygen vacancies) can have a low carrier density. Such an oxide semiconductor is referred to as a highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor. A CAAC-OS has a low impurity concentration and a low density of defect states. That is, a CAAC-OS and an nc-OS are likely to be highly purified intrinsic or substantially highly purified intrinsic oxide semiconductors. Thus, a transistor including a CAAC-OS rarely has negative threshold voltage (is rarely normally on). The highly purified intrinsic or substantially highly purified intrinsic oxide semiconductor has few carrier traps. An electric charge trapped by the carrier traps in the oxide semiconductor takes a long time to be released. The trapped electric charge may behave like a fixed electric charge. Thus, the transistor that includes the oxide semiconductor having a high impurity concentration and a high density of defect states might have unstable electrical characteristics. However, a transistor including a CAAC-OS has small variations in electrical characteristics and high reliability.

Since the CAAC-OS has a low density of defect states, carriers generated by light irradiation or the like are less likely to be trapped in defect states. Thus, with the use of the CAAC-OS in a transistor, changes in the electrical characteristics of the transistor due to irradiation with visible light or ultraviolet light are small.

<Microcrystalline Oxide Semiconductor>

Next, a microcrystalline oxide semiconductor is described.

A microcrystalline oxide semiconductor has a region in which a crystal part is observed and a region in which a crystal part is not observed clearly in a high-resolution TEM image. In most cases, the size of a crystal part included in the microcrystalline oxide semiconductor is greater than or equal to 1 nm and less than or equal to 100 nm, or greater than or equal to 1 nm and less than or equal to 10 nm. An oxide semiconductor including a nanocrystal that is a microcrystal with a size greater than or equal to 1 nm and less than or equal to 10 nm, or a size greater than or equal to 1 nm and less than or equal to 3 nm is specifically referred to as a nanocrystalline oxide semiconductor (nc-OS). In a high-resolution TEM image of the nc-OS, for example, a grain boundary is not clearly observed in some cases. Note that there is a possibility that the origin of the nanocrystal is the same as that of a pellet in a CAAC-OS. Thus, a crystal part of the nc-OS may be referred to as a pellet in the following description.

In the nc-OS, a microscopic region (for example, a region with a size greater than or equal to 1 nm and less than or equal to 10 nm, in particular, a region with a size greater than or equal to 1 nm and less than or equal to 3 nm) has a periodic atomic arrangement. There is no regularity of crystal orientation between different pellets in the nc-OS. Thus, the orientation of the whole film is not observed. Accordingly, in some cases, the nc-OS cannot be distinguished from an amorphous oxide semiconductor, depending on a certain analysis method. For example, when the nc-OS is subjected to structural analysis by an out-of-plane method with an XRD apparatus using an X-ray having a diameter larger than the size of a pellet, a peak which shows a crystal plane does not appear. Furthermore, a diffraction pattern like a halo pattern is observed when the nc-OS is subjected to electron diffraction using an electron beam with a probe diameter (e.g., 50 nm or larger) that is larger than the size of a pellet (the electron diffraction is also referred to as selected-area electron diffraction). Meanwhile, spots appear in a nanobeam electron diffraction pattern of the nc-OS when an electron beam having a probe diameter close to or smaller than the size of a pellet is applied. Moreover, in a nanobeam electron diffraction pattern of the nc-OS, regions with high luminance in a circular (ring) pattern are shown in some cases. Also in a nanobeam electron diffraction pattern of the nc-OS, a plurality of spots are observed in a ring-like region in some cases.

Since there is no regularity of crystal orientation between the pellets (nanocrystals) as mentioned above, the nc-OS can also be referred to as an oxide semiconductor including random aligned nanocrystals (RANC) or an oxide semiconductor including non-aligned nanocrystals (NANC).

The nc-OS is an oxide semiconductor that has higher regularity than an amorphous oxide semiconductor, and thus has a lower density of defect states than the amorphous oxide semiconductor. Note that there is no regularity of crystal orientation between different pellets in the nc-OS. Therefore, the nc-OS has a higher density of defect states than the CAAC-OS.

<Amorphous Oxide Semiconductor>

Next, an amorphous oxide semiconductor is described.

The amorphous oxide semiconductor is an oxide semiconductor having disordered atomic arrangement and no crystal part and exemplified by an oxide semiconductor which exists in an amorphous state as quartz.

In a high-resolution TEM image of the amorphous oxide semiconductor, crystal parts cannot be observed.

When the amorphous oxide semiconductor is subjected to structural analysis by an out-of-plane method with an XRD apparatus, a peak showing a crystal plane does not appear. A halo pattern is observed when the amorphous oxide semiconductor is subjected to electron diffraction. Furthermore, a spot is not observed and only a halo pattern appears when the amorphous oxide semiconductor is subjected to nanobeam electron diffraction.

There are various understandings of an amorphous structure. For example, a structure whose atomic arrangement does not have ordering at all is called a completely amorphous structure. Meanwhile, a structure that has ordering until the nearest neighbor atomic distance or the second-nearest neighbor atomic distance but does not have long-range ordering is also called an amorphous structure. Therefore, the strictest definition does not permit an oxide semiconductor to be called an amorphous oxide semiconductor as long as even a negligible degree of ordering is present in an atomic arrangement. At least an oxide semiconductor having long-term ordering cannot be called an amorphous oxide semiconductor. Accordingly, because of the presence of a crystal part, for example, a CAAC-OS and an nc-OS cannot be called an amorphous oxide semiconductor or a completely amorphous oxide semiconductor.

<Amorphous-Like Oxide Semiconductor>

Note that an oxide semiconductor may have a structure intermediate between the nc-OS and the amorphous oxide semiconductor. The oxide semiconductor having such a structure is specifically referred to as an amorphous-like oxide semiconductor (a-like OS).

In a high-resolution TEM image of the a-like OS, a void may be observed. Furthermore, in the high-resolution TEM image, there are a region where a crystal part is clearly observed and a region where a crystal part is not observed.

The a-like OS has an unstable structure because it includes a void. To verify that an a-like OS has an unstable structure as compared with a CAAC-OS and an nc-OS, a change in structure caused by electron irradiation is described below.

An a-like OS (sample A), an nc-OS (sample B), and a CAAC-OS (sample C) are prepared as samples subjected to electron irradiation. Each of the samples is an In—Ga—Zn oxide.

First, a high-resolution cross-sectional TEM image of each sample is obtained. The high-resolution cross-sectional TEM images show that all the samples have crystal parts.

Note that which part is regarded as a crystal part is determined as follows. It is known that a unit cell of the InGaZnO₄ crystal has a structure in which nine layers including three In—O layers and six Ga—Zn—O layers are stacked in the c-axis direction. The distance between the adjacent layers is equivalent to the lattice spacing on the (009) plane (also referred to as d value). The value is calculated to be 0.29 nm from crystal structural analysis. Accordingly, a portion where the lattice spacing between lattice fringes is greater than or equal to 0.28 nm and less than or equal to 0.30 nm is regarded as a crystal part of InGaZnO₄. Each of the lattice fringes corresponds to the a-b plane of the InGaZnO₄ crystal.

FIG. 43 shows change in the average size of crystal parts (at 22 points to 45 points) in each sample. Note that the crystal part size corresponds to the length of a lattice fringe. FIG. 43 indicates that the crystal part size in the a-like OS increases with an increase in the cumulative electron dose. Specifically, as shown by (1) in FIG. 43, a crystal part of approximately 1.2 nm (also referred to as an initial nucleus) at the start of TEM observation grows to a size of approximately 2.6 nm at a cumulative electron dose of 4.2×10⁸ e⁻/nm². In contrast, the crystal part size in the nc-OS and the CAAC-OS shows little change from the start of electron irradiation to a cumulative electron dose of 4.2×10⁸ e⁻/nm². Specifically, as shown by (2) and (3) in FIG. 43, the average crystal sizes in an nc-OS and a CAAC-OS are approximately 1.4 nm and approximately 2.1 nm, respectively, regardless of the cumulative electron dose.

In this manner, growth of the crystal part in the a-like OS is induced by electron irradiation. In contrast, in the nc-OS and the CAAC-OS, growth of the crystal part is hardly induced by electron irradiation. Therefore, the structure of the a-like OS is more unstable than those of the nc-OS and the CAAC-OS.

The a-like OS has a lower density than the nc-OS and the CAAC-OS because it includes a void. Specifically, the density of the a-like OS is higher than or equal to 78.6% and lower than 92.3% of the density of the single crystal oxide semiconductor having the same composition. The density of each of the nc-OS and the CAAC-OS is higher than or equal to 92.3% and lower than 100% of the density of the single crystal oxide semiconductor having the same composition. Note that it is difficult to deposit an oxide semiconductor layer having a density of lower than 78% of the density of the single crystal oxide semiconductor layer.

For example, in the case of an oxide semiconductor having an atomic ratio of In to Ga and of 1:1:1, the density of single crystal InGaZnO₄ with a rhombohedral crystal structure is 6.357 g/cm³. Accordingly, in the case of the oxide semiconductor having an atomic ratio of In to Ga and Zn of 1:1:1, the density of the a-like OS is higher than or equal to 5.0 g/cm³ and lower than 5.9 g/cm³. For example, in the case of the oxide semiconductor having an atomic ratio of In to Ga and Zn of 1:1:1, the density of each of the nc-OS and the CAAC-OS is higher than or equal to 5.9 g/cm³ and lower than 6.3 g/cm³.

Note that single crystals with the same composition do not exist in some cases. In that case, single crystal oxide semiconductors with different compositions are combined at an adequate ratio, which makes it possible to calculate density equivalent to that of a single crystal oxide semiconductor with the desired composition. The density of a single crystal oxide semiconductor having the desired composition can be calculated using a weighted average according to the combination ratio of the single crystal oxide semiconductors with different compositions. Note that it is preferable to use as few kinds of single crystal oxide semiconductors as possible to calculate the density.

As described above, oxide semiconductors have various structures and various properties. Note that an oxide semiconductor may be a stacked film including two or more films of an amorphous oxide semiconductor, an a-like OS, a microcrystalline oxide semiconductor, and a CAAC-OS, for example.

<Deposition Model>

Examples of deposition models of a CAAC-OS and an nc-OS are described below.

FIG. 44A is a schematic view of the inside of a deposition chamber where a CAAC-OS is deposited by a sputtering method.

A target 5130 is attached to a backing plate. A plurality of magnets are provided to face the target 5130 with the backing plate positioned therebetween. The plurality of magnets generate a magnetic field. A sputtering method in which the disposition speed is increased by utilizing a magnetic field of magnets is referred to as a magnetron sputtering method.

A substrate 5120 is placed to face the target 5130, and the distance d (also referred to as a target □ substrate distance (T□S distance)) is greater than or equal to 0.01 m and less than or equal to 1 m, preferably greater than or equal to 0.02 m and less than or equal to 0.5 m. The deposition chamber is mostly filled with a deposition gas (e.g., an oxygen gas, an argon gas, or a mixed gas containing oxygen at 5 vol. % or higher) and the pressure in the deposition chamber is controlled to be higher than or equal to 0.01 Pa and lower than or equal to 100 Pa, preferably higher than or equal to 0.1 Pa and lower than or equal to 10 Pa. Here, discharge starts by application of a voltage at a constant value or higher to the target 5130, and plasma is observed. The magnetic field forms a high-density plasma region in the vicinity of the target 5130. In the high-density plasma region, the deposition gas is ionized, so that an ion 5101 is generated. Examples of the ion 5101 include an oxygen cation (O⁺) and an argon cation (Ar⁺).

Here, the target 5130 has a polycrystalline structure which includes a plurality of crystal grains and in which a cleavage plane exists in at least one crystal grain. FIG. 45A shows a structure of an InGaZnO₄ crystal included in the target 5130 as an example. Note that FIG. 45A shows a structure of the case where the InGaZnO₄ crystal is observed from a direction parallel to the b-axis. FIG. 45A indicates that oxygen atoms in a Ga—Zn—O layer are positioned close to those in an adjacent Ga—Zn—O layer. The oxygen atoms have negative charge, whereby repulsive force is generated between the two Ga—Zn—O layers. As a result, the InGaZnO₄ crystal has a cleavage plane between the two adjacent Ga—Zn—O layers.

The ion 5101 generated in the high-density plasma region is accelerated toward the target 5130 side by an electric field, and then collides with the target 5130. At this time, a pellet 5100 a and a pellet 5100 b which are flat-plate-like (pellet-like) sputtered particles are separated and sputtered from the cleavage plane. Note that structures of the pellet 5100 a and the pellet 5100 b may be distorted by an impact of collision of the ion 5101.

The pellet 5100 a is a flat-plate-like (pellet-like) sputtered particle having a triangle plane, e.g., regular triangle plane. The pellet 5100 b is a flat-plate-like (pellet-like) sputtered particle having a hexagon plane, e.g., regular hexagon plane. Note that flat-plate-like (pellet-like) sputtered particles such as the pellet 5100 a and the pellet 5100 b are collectively called pellets 5100. The shape of a flat plane of the pellet 5100 is not limited to a triangle or a hexagon. For example, the flat plane may have a shape formed by combining two or more triangles. For example, a quadrangle (e.g., rhombus) may be formed by combining two triangles (e.g., regular triangles).

The thickness of the pellet 5100 is determined depending on the kind of deposition gas and the like. The thicknesses of the pellets 5100 are preferably uniform; the reasons thereof are described later. In addition, the sputtered particle preferably has a pellet shape with a small thickness as compared to a dice shape with a large thickness. For example, the thickness of the pellet 5100 is greater than or equal to 0.4 nm and less than or equal to 1 nm, preferably greater than or equal to 0.6 nm and less than or equal to 0.8 nm. In addition, the width of the pellet 5100 is, for example, greater than or equal to 1 nm and less than or equal to 3 nm, preferably greater than or equal to 1.2 nm and less than or equal to 2.5 nm. The pellet 5100 corresponds to the initial nucleus in the description of (1) in FIG. 43. For example, when the ion 5101 collides with the target 5130 including an In—Ga—Zn oxide, the pellet 5100 that includes three layers of a Ga—Zn—O layer, an In—O layer, and a Ga—Zn—O layer as shown in FIG. 45B is separated. FIG. 45C shows the structure of the separated pellet 5100 observed from a direction parallel to the c-axis. The pellet 5100 has a nanometer-sized sandwich structure including two Ga—Zn—O layers (pieces of bread) and an In—O layer (filling).

The pellet 5100 may receive a charge when passing through the plasma, so that side surfaces thereof are negatively or positively charged. In the pellet 5100, an oxygen atom positioned on its side surface may be negatively charged. In this manner, when the side surfaces are charged with the same polarity, charges repel each other, and accordingly, the pellet 5100 can maintain a flat-plate shape. In the case where a CAAC-OS is an In—Ga—Zn oxide, there is a possibility that an oxygen atom bonded to an indium atom is negatively charged. There is another possibility that an oxygen atom bonded to an indium atom, a gallium atom, or a zinc atom is negatively charged. In addition, the pellet 5100 may grow by being bonded with an indium atom, a gallium atom, a zinc atom, an oxygen atom, or the like when passing through plasma. A difference in size between (2) and (1) in FIG. 43 corresponds to the amount of growth in plasma. Here, in the case where the temperature of the substrate 5120 is at around room temperature, the pellet 5100 on the substrate 5120 hardly grows; thus, an nc-OS is formed (see FIG. 44B). An nc-OS can be deposited when the substrate 5120 has a large size because the deposition of an nc-OS can be carried out at room temperature. Note that in order that the pellet 5100 grows in plasma, it is effective to increase deposition power in sputtering. High deposition power can stabilize the structure of the pellet 5100.

As illustrated in FIGS. 44A and 44B, the pellet 5100 flies like a kite in plasma and flutters up to the substrate 5120. Since the pellets 5100 are charged, when the pellet 5100 gets close to a region where another pellet 5100 has already been deposited, repulsion is generated. Here, above the substrate 5120, a magnetic field in a direction parallel to the top surface of the substrate 5120 (also referred to as a horizontal magnetic field) is generated. A potential difference is given between the substrate 5120 and the target 5130, and accordingly, current flows from the substrate 5120 toward the target 5130. Thus, the pellet 5100 is given a force (Lorentz force) on the top surface of the substrate 5120 by an effect of the magnetic field and the current. This is explainable with Fleming's left-hand rule.

The mass of the pellet 5100 is larger than that of an atom. Therefore, to move the pellet 5100 over the top surface of the substrate 5120, it is important to apply some force to the pellet 5100 from the outside. One kind of the force may be force which is generated by the action of a magnetic field and current. To apply a sufficient force to the pellet 5100 so that the pellet 5100 moves over a top surface of the substrate 5120, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 10 G or higher, preferably 20 G or higher, further preferably 30 G or higher, still further preferably 50 G or higher. Alternatively, it is preferable to provide, on the top surface, a region where the magnetic field in a direction parallel to the top surface of the substrate 5120 is 1.5 times or higher, preferably twice or higher, further preferably 3 times or higher, still further preferably 5 times or higher as high as the magnetic field in a direction perpendicular to the top surface of the substrate 5120.

At this time, the magnets and the substrate 5120 are moved or rotated relatively, whereby the direction of the horizontal magnetic field on the top surface of the substrate 5120 continues to change. Thus, the pellet 5100 can be moved in various directions on the top surface of the substrate 5120 by receiving forces in various directions.

Furthermore, as illustrated in FIG. 44A, when the substrate 5120 is heated, the resistance between the pellet 5100 and the substrate 5120 due to friction or the like is low. As a result, the pellet 5100 glides above the top surface of the substrate 5120. The glide of the pellet 5100 is caused in a state where the flat plane faces the substrate 5120. Then, when the pellet 5100 reaches the side surface of another pellet 5100 that has been already deposited, the side surfaces of the pellets 5100 are bonded. At this time, the oxygen atom on the side surface of the pellet 5100 is released. With the released oxygen atom, oxygen vacancies in a CAAC-OS is filled in some cases; thus, the CAAC-OS has a low density of defect states. Note that the temperature of the top surface of the substrate 5120 is, for example, higher than or equal to 100° C. and lower than 500° C., higher than or equal to 150° C. and lower than 450° C., or higher than or equal to 170° C. and lower than 400° C. Hence, even when the substrate 5120 has a large size, it is possible to deposit a CAAC-OS.

Furthermore, the pellet 5100 is heated on the substrate 5120, whereby atoms are rearranged, and the structure distortion caused by the collision of the ion 5101 can be reduced. The pellet 5100 whose structure distortion is reduced is substantially single crystal. Even when the pellets 5100 are heated after being bonded, expansion and contraction of the pellet 5100 itself hardly occur, which is caused by turning the pellet 5100 into substantially single crystal. Thus, formation of defects such as a grain boundary due to expansion of a space between the pellets 5100 can be prevented, and accordingly, generation of crevasses can be prevented.

The CAAC-OS does not have a structure like a board of a single crystal oxide semiconductor but has arrangement with a group of pellets 5100 (nanocrystals) like stacked bricks or blocks. Furthermore, a grain boundary does not exist between the pellets 5100. Therefore, even when deformation such as shrink occurs in the CAAC-OS owing to heating during deposition, heating or bending after deposition, it is possible to relieve local stress or release distortion. Therefore, this structure is suitable for a flexible semiconductor device. Note that the nc-OS has arrangement in which pellets 5100 (nanocrystals) are randomly stacked.

When the target 5130 is sputtered with the ion 5101, in addition to the pellets 5100, zinc oxide or the like may be separated. The zinc oxide is lighter than the pellet and thus reaches the top surface of the substrate 5120 before the pellet 5100. As a result, the zinc oxide forms a zinc oxide layer 5102 with a thickness greater than or equal to 0.1 nm and less than or equal to 10 nm, greater than or equal to 0.2 nm and less than or equal to 5 nm, or greater than or equal to 0.5 nm and less than or equal to 2 nm. FIGS. 46A to 46D are cross-sectional schematic views.

As illustrated in FIG. 46A, a pellet 5105 a and a pellet 5105 b are deposited over the zinc oxide layer 5102. Here, side surfaces of the pellet 5105 a and the pellet 5105 b are in contact with each other. In addition, a pellet 5105 c is deposited over the pellet 5105 b, and then glides over the pellet 5105 b. Furthermore, a plurality of particles 5103 separated from the target together with the zinc oxide is crystallized by heating of the substrate 5120 to form a region 5105 a 1 on another side surface of the pellet 5105 a. Note that the plurality of particles 5103 may contain oxygen, zinc, indium, gallium, or the like.

Then, as illustrated in FIG. 46B, the region 5105 a 1 grows to part of the pellet 5105 a to form a pellet 5105 a 2. In addition, a side surface of the pellet 5105 c is in contact with another side surface of the pellet 5105 b.

Next, as illustrated in FIG. 46C, a pellet 5105 d is deposited over the pellet 5105 a 2 and the pellet 5105 b, and then glides over the pellet 5105 a 2 and the pellet 5105 b. Furthermore, a pellet 5105 e glides toward another side surface of the pellet 5105 c over the zinc oxide layer 5102.

Then, as illustrated in FIG. 46D, the pellet 5105 d is placed so that a side surface of the pellet 5105 d is in contact with a side surface of the pellet 5105 a 2. Furthermore, a side surface of the pellet 5105 e is in contact with another side surface of the pellet 5105 c. A plurality of particles 5103 separated from the target 5130 together with the zinc oxide is crystallized by heating of the substrate 5120 to form a region 5105 d 1 on another side surface of the pellet 5105 d.

As described above, deposited pellets are placed to be in contact with each other and then growth is caused at side surfaces of the pellets, whereby a CAAC-OS is formed over the substrate 5120. Therefore, each pellet of the CAAC-OS is larger than that of the nc-OS. A difference in size between (3) and (2) in FIG. 43 corresponds to the amount of growth after deposition.

When spaces between pellets are extremely small, the pellets may form a large pellet. The large pellet has a single crystal structure. For example, the size of the pellet may be greater than or equal to 10 nm and less than or equal to 200 nm, greater than or equal to 15 nm and less than or equal to 100 nm, or greater than or equal to 20 nm and less than or equal to 50 nm, when seen from above. In this case, in an oxide semiconductor used for a minute transistor, a channel formation region might be fit inside the large pellet. Therefore, the region having a single crystal structure can be used as the channel formation region. Furthermore, when the size of the pellet is increased, the region having a single crystal structure can be used as the channel formation region, the source region, and the drain region of the transistor.

In this manner, when the channel formation region or the like of the transistor is formed in a region having a single crystal structure, the frequency characteristics of the transistor can be increased in some cases.

As illustrated in such a model, the pellets 5100 are considered to be deposited on the substrate 5120. Thus, a CAAC-OS can be deposited even when a formation surface does not have a crystal structure; therefore, a growth mechanism in this case is different from epitaxial growth. In addition, laser crystallization is not needed for formation of a CAAC-OS, and a uniform film can be formed even over a large-sized glass substrate or the like. For example, even when the top surface (formation surface) of the substrate 5120 has an amorphous structure (e.g., the top surface is formed of amorphous silicon oxide), a CAAC-OS can be formed.

In addition, it is found that in formation of the CAAC-OS, the pellets 5100 are arranged in accordance with the top surface shape of the substrate 5120 that is the formation surface even when the formation surface has unevenness. For example, in the case where the top surface of the substrate 5120 is flat at the atomic level, the pellets 5100 are arranged so that flat planes parallel to the a-b plane face downwards. In the case where the thicknesses of the pellets 5100 are uniform, a layer with a uniform thickness, flatness, and high crystallinity is formed. By stacking n layers (n is a natural number), the CAAC-OS can be obtained.

In the case where the top surface of the substrate 5120 has unevenness, a CAAC-OS in which n layers (n is a natural number) in each of which the pellets 5100 are arranged along the unevenness are stacked is formed. Since the substrate 5120 has unevenness, a gap is easily generated between in the pellets 5100 in the CAAC-OS in some cases. Note that owing to intermolecular force, the pellets 5100 are arranged so that a gap between the pellets is as small as possible even on the unevenness surface. Therefore, even when the formation surface has unevenness, a CAAC-OS with high crystallinity can be obtained.

Since the CAAC-OS film is deposited in accordance with such a model, the sputtered particle preferably has a pellet shape with a small thickness. Note that when the sputtered particles has a dice shape with a large thickness, planes facing the substrate 5120 vary, which may lead to formation of a film whose thickness or crystal alignment is not uniformed.

According to the deposition model described above, a CAAC-OS with high crystallinity can be formed even on a film formation surface with an amorphous structure.

The semiconductor device of one embodiment of the present invention can be formed using an oxide semiconductor film having any of the above structures.

The structure and method described in this embodiment can be combined as appropriate with any of the other structures and methods described in the other embodiments.

Embodiment 5

In this embodiment, an example of a display device that includes any of the transistors described above and has a display function is described below with reference to FIGS. 27A and 27B, FIG. 28, FIG. 29, and FIG. 30.

FIG. 27A is a top view of an example of a display device. A display device 700 illustrated in FIG. 27A includes a pixel portion 702 provided over a first substrate 701; a source driver circuit portion 704 and a gate driver circuit portion 706 provided over the first substrate 701; a sealant 712 provided to surround the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706; and a second substrate 705 provided to face the first substrate 701. The first substrate 701 and the second substrate 705 are sealed with the sealant 712. That is, the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 are sealed with the first substrate 701, the sealant 712, and the second substrate 705. Although not illustrated in FIG. 27A, a display element is provided between the first substrate 701 and the second substrate 705.

In the display device 700, a flexible printed circuit (FPC) terminal portion 708 electrically connected to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 is provided in a region different from the region which is surrounded by the sealant 712 and positioned over the first substrate 701. Furthermore, an FPC 716 is connected to the FPC terminal portion 708, and a variety of signals and the like are supplied to the pixel portion 702, the source driver circuit portion 704, and the gate driver circuit portion 706 through the FPC 716. Furthermore, a signal line 710 is connected to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708. Various signals and the like are applied to the pixel portion 702, the source driver circuit portion 704, the gate driver circuit portion 706, and the FPC terminal portion 708 via the signal line 710 from the FPC 716.

FIG. 27B is a top view of an example of a display device. In a display device 800 illustrated in FIG. 27B, a pixel portion 802 is used instead of the pixel portion 702 in the display device 700 illustrated in FIG. 27A.

A plurality of gate driver circuit portions 706 may be provided in each of the display devices 700 and 800. An example in which the source driver circuit portion 704 and the gate driver circuit portion 706 are formed over the first substrate 701 where the pixel portion 702 or 802 is also formed in the display devices 700 and 800 is described; however, one embodiment of the present invention is not limited to the structure. For example, only the gate driver circuit portion 706 may be formed over the first substrate 701 or only the source driver circuit portion 704 may be formed over the first substrate 701. In this case, a substrate where a source driver circuit, a gate driver circuit, or the like is faulted (e.g., a driver circuit substrate formed using a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted on the first substrate 701.

There is no particular limitation on the connection method of a separately formed driver circuit substrate; a chip on glass (COG) method, a wire bonding method, a tape automated bonding (TAB) method, or the like can be used. Note that the display device in this specification means an image display device, a display device, or a light source (including a lighting device or the like). Furthermore, the display device also includes the following modules in its category: a module to which a connector such as an FPC or a tape carrier package (TCP) is attached; a module having a TCP at the tip of which a printed wiring board is provided; and a module in which a driver circuit substrate or an integrated circuit (IC) is directly mounted on a display element by a COG method.

The pixel portions 702 and 802, the source driver circuit portion 704, and the gate driver circuit portion 706 included in the display devices 700 and 800 include a plurality of transistors. As the plurality of transistors, any of the transistors that are the semiconductor devices of embodiments of the present invention can be used.

Note that the display device 700 includes a liquid crystal element as a display element, and the display device 800 includes a light-emitting element as a display element.

Note that a display element, a display device that is a device including a display element, a light-emitting element, and a light-emitting device that is a device including a light-emitting element can employ various modes and can include various elements. A display element, a display device, a light-emitting element, or a light-emitting device includes, for example, at least one of an electroluminescence (EL) element (e.g., an EL element including organic and inorganic materials, an organic EL element, or an inorganic EL element), an LED (e.g., a white LED, a red LED, a green LED, or a blue LED), a transistor (a transistor that emits light depending on current), an electron emitter, a liquid crystal element, electronic ink, an electrophoretic element, a grating light valve (GLV), a plasma display panel (PDP), a display element using micro electro mechanical system (MEMS), a digital micromirror device (DMD), a digital micro shutter (DMS), MIRASOL (registered trademark), an interferometric modulator display (IMOD) element, a MEMS shutter display element, an optical-interference-type MEMS display element, an electrowetting element, a piezoelectric ceramic display, and a display element including a carbon nanotube. In addition, a display medium whose contrast, luminance, reflectance, transmittance, or the like is changed by electric action or magnetic action may be included. Examples of display devices including EL elements include an EL display. Examples of display devices including electron emitters are a field emission display (FED) and an SED-type flat panel display (SED: surface-conduction electron-emitter display). Examples of display devices including liquid crystal elements include a liquid crystal display (e.g., a transmissive liquid crystal display, a transflective liquid crystal display, a reflective liquid crystal display, a direct-view liquid crystal display, or a projection liquid crystal display). An example of a display device including electronic ink or electrophoretic elements is electronic paper. In the case of a transflective liquid crystal display or a reflective liquid crystal display, some of or all of pixel electrodes function as reflective electrodes. For example, some or all of pixel electrodes are formed to contain aluminum, silver, or the like. In such a case, a memory circuit such as an SRAM can be provided under the reflective electrodes, leading to lower power consumption.

The display device 700 and the display device 800 are described in detail with reference to FIG. 28 and FIG. 29. Common portions between the display device 700 and the display device 800 are described first, and then different portions are described.

<Common Portions in Display Device>

FIG. 28 is a cross-sectional view taken along the dashed dotted line Q-R in FIG. 27A.

FIG. 29 is a cross-sectional view taken along the dashed dotted line V-W in FIG. 27B.

The display devices 700 and 800 illustrated in FIG. 28 and FIG. 29 include a lead wiring portion 711, the pixel portion 702 or 802, the source driver circuit portion 704, and the FPC terminal portion 708. Note that the lead wiring portion 711 includes the signal line 710.

The signal line 710 included in the lead wiring portion 711 is formed in the same process as conductive films serving as gate electrodes, source electrodes, and drain electrodes of transistors 750 and 752. Note that the signal line 710 may be formed using a conductive film that is formed in a different process from the gate electrodes, the source electrode, and the drain electrodes of the transistors 750 and 752, for example, a conductive film serving as a lead wiring.

The FPC terminal portion 708 includes a connection electrode 760, an anisotropic conductive film 780, and the FPC 716. Note that the connection electrode 760 is formed in the same process as the conductive films functioning as the source electrode layer and the drain electrode layer of the transistor 750. The connection electrode 760 is electrically connected to a terminal included in the FPC 716 through the anisotropic conductive film 780.

The display devices 700 and 800 illustrated in FIG. 28 and FIG. 29 are examples in which the transistor 750 is provided in the pixel portions 702 and 802, and a transistor 752 is provided in the source driver circuit portion 704. The transistors 750 and 752 each have a structure similar to that of the transistor 150 illustrated in FIGS. 1A to 1C. Note that the structures of the transistors 750 and 752 are not limited to the structure of the transistor 150. For example, the transistors 750 and 752 may have any of the structures of the transistors 151 to 154, the transistors 190 to 194, the transistor 150A, the transistor 190A, and the transistor 190B.

In the transistor used in this embodiment, which includes an oxide semiconductor film which is highly purified and in which formation of oxygen vacancies is suppressed, the current in an off state (off-state current) can be made low. Accordingly, an electrical signal such as an image signal can be held for a longer period, and a writing interval can be set longer in an on state. Accordingly, frequency of refresh operation can be reduced, which leads to an effect of suppressing power consumption.

The transistor used in this embodiment, which includes an oxide semiconductor film which is highly purified and in which formation of oxygen vacancy is suppressed, can have relatively high field-effect mobility and thus can operate at high speed. For example, with such a transistor which can operate at high speed used for a liquid crystal display device, a switching transistor in a pixel portion and a driver transistor in a driver circuit portion can be formed over one substrate. That is, a semiconductor device formed using a silicon wafer or the like is not additionally needed as a driver circuit, by which the number of components of the semiconductor device can be reduced. In addition, the transistor which can operate at high speed can be used also in the pixel portion, whereby a high-quality image can be provided.

A wiring containing a copper element can be used for the transistor of the pixel portion and the signal line connected to the transistor used in the driver circuit portion. Therefore, in the display device of one embodiment of the present invention, signal delay or the like due to wiring resistance is reduced, which enables display on a large screen.

Note that in this embodiment, the transistor 750 included in the pixel portion 702 or 802 and the transistor 752 included in the source driver circuit portion 704 have in the same size; however, this embodiment is not limited to this. The sizes (L/W) or the number of the transistors used in the pixel portion 702 and the source driver circuit portion 704 may vary as appropriate. The gate driver circuit portion 706 is not illustrated in FIG. 28 and FIG. 29; however, the gate driver circuit portion 706 can have a structure similar to that of the source driver circuit portion 704.

Furthermore, in FIG. 28 and FIG. 29, a planarization insulating film 770 is provided over insulating films 764 and 766 included in the transistor 750 and the transistor 752.

The insulating films 764 and 766 can be formed using a material and a method similar to those of the insulating films 116 and 118 described in the above embodiments.

The planarization insulating film 770 can be formed using a heat-resistant organic material, such as a polyimide resin, an acrylic resin, a polyimide amide resin, a benzocyclobutene resin, a polyamide resin, or an epoxy resin. Note that the planarization insulating film 770 may be formed by stacking a plurality of insulating films formed using these materials. Alternatively, a structure without the planarization insulating film 770 may be employed.

A conductive film 772 or a conductive film 844 is connected to one of the conductive films functioning as the source electrode and the drain electrode included in the transistor 750. The conductive films 772 and 844 are each formed over the planarization insulating film 770 to function as a pixel electrode, i.e., one electrode of the display portion. As the conductive film 772, a conductive film which transmits visible light is preferably used. For example, the conductive film is preferably formed using a material including one of indium (In), zinc (Zn), and tin (Sn). As the conductive film 844, a reflective conductive film is preferably used.

<Structure Example 1 of Display Device Using Liquid Crystal Element as Display Element>

The display device 700 illustrated in FIG. 28 includes a liquid crystal element 775. The liquid crystal element 775 includes the conductive film 772, a conductive film 774, and a liquid crystal layer 776. The conductive film 774 is provided on the second substrate 705 side and functions as a counter electrode. The display device 700 in FIG. 28 is capable of displaying an image in such a manner that transmission or non-transmission is controlled by change in the alignment state of the liquid crystal layer 776 depending on a voltage applied to the conductive film 772 and the conductive film 774.

Although not illustrated in FIG. 28, an alignment film may be provided on a side of the conductive film 772 in contact with the liquid crystal layer 776 and on a side of the conductive film 774 in contact with the liquid crystal layer 776. In addition, although not illustrated in FIG. 28, a color filter (a coloring film), a black matrix (a light-shielding film), an optical member (an optical substrate) such as a polarizing member, a retardation member, or an anti-reflection member, or the like may be provided as appropriate. For example, circular polarization may be employed by using a polarizing substrate and a retardation substrate. In addition, a backlight, a sidelight, or the like may be used as a light source.

For example, a glass substrate can be used as the first substrate 701 and the second substrate 705. As the first substrate 701 and the second substrate 705, a flexible substrate may be used. Examples of the flexible substrate include a plastic substrate.

A spacer 778 is provided between the first substrate 701 and the second substrate 705. The spacer 778 is a columnar spacer obtained by selective etching of an insulating film and is provided to control the thickness (cell gap) of the liquid crystal layer 776. Note that a spherical spacer may be used as the spacer 778.

In the case where a liquid crystal element is used as the display element, a thermotropic liquid crystal, a low-molecular liquid crystal, a high-molecular liquid crystal, a polymer dispersed liquid crystal, a ferroelectric liquid crystal, an anti-ferroelectric liquid crystal, or the like can be used. Such a liquid crystal material exhibits a cholesteric phase, a smectic phase, a cubic phase, a chiral nematic phase, an isotropic phase, or the like depending on conditions.

Alternatively, in the case of employing a horizontal electric field mode, a liquid crystal exhibiting a blue phase for which an alignment film is unnecessary may be used. A blue phase is one of liquid crystal phases, which is generated just before a cholesteric phase changes into an isotropic phase while temperature of cholesteric liquid crystal is increased. Since the blue phase appears only in a narrow temperature range, a liquid crystal composition in which several weight percent or more of a chiral material is mixed is used for the liquid crystal layer in order to improve the temperature range. The liquid crystal composition containing a liquid crystal showing a blue phase and a chiral material has a short response time and optical isotropy, which eliminates the need for an alignment process. In addition, when the liquid crystal composition that includes a liquid crystal exhibiting a blue phase and a chiral material is used, an alignment film does not need to be provided, which eliminates the need for rubbing treatment. Thus, electrostatic discharge damage caused by rubbing treatment can be prevented and defects and damage of the liquid crystal display device in the manufacturing process can be reduced.

In the case where a liquid crystal element is used as the display element, a twisted nematic (TN) mode, an in-plane-switching (IPS) mode, a fringe field switching (FFS) mode, an axially symmetric aligned micro-cell (ASM) mode, an optical compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, or the like can be used.

A normally black liquid crystal display device such as a transmissive liquid crystal display device utilizing a vertical alignment (VA) mode may be used. There are some examples of a vertical alignment mode; for example, a multi-domain vertical alignment (MVA) mode, a patterned vertical alignment (PVA) mode, an ASV mode, or the like can be employed.

As a display method in the pixel portion 702, a progressive method, an interlace method, or the like can be employed. Furthermore, color elements controlled in a pixel at the time of color display are not limited to three colors: R, G, and B (R, G, and B correspond to red, green, and blue, respectively). For example, four pixels of a R pixel, a G pixel, a B pixel, and a W (white) pixel may be included. Alternatively, a color element may be composed of two colors among R, G, and B as in PenTile layout. The two colors may differ among color elements. Alternatively, one or more colors of yellow, cyan, magenta, and the like may be added to RGB. Note that, the sizes of display regions may be different between respective dots of color elements. Embodiments of the disclosed invention are not limited to a display device for color display; the disclosed invention can also be applied to a display device for monochrome display.

<Display Device Using Light-Emitting Element as Display Element>

The display device 800 illustrated in FIG. 29 includes a light-emitting element 880. The light-emitting element 880 includes the conductive film 844, an EL layer 846, and a conductive film 848. The display device 800 is capable of displaying an image by light emission from the EL layer 846 included in the light-emitting element 880.

In the display device 800 illustrated in FIG. 29, the insulating film 830 is provided over the planarization insulating film 770 and the conductive film 844. The insulating film 830 covers part of the conductive film 844. Note that the light-emitting element 880 has a top emission structure. Therefore, the conductive film 848 has a light-transmitting property and transmits light emitted from the EL layer 846. Although the top-emission structure is described as an example in this embodiment, one embodiment of the present invention is not limited thereto. A bottom-emission structure in which light is emitted to the conductive film 844 side, or a dual-emission structure in which light is emitted to both the conductive film 844 side and the conductive film 848 side may be employed.

The coloring film 836 is provided to overlap the light-emitting element 880, and the light-blocking film 838 is provided to overlap the insulating film 830 and to be included in the lead wiring portion 711 and in the source driver circuit portion 704. The coloring film 836 and the light-blocking film 838 are covered with the insulating film 834. A space between the light-emitting element 880 and the insulating film 834 is filled with the sealing film 832. Although the structure of the display device 800 with the coloring film 836 is described in this embodiment, one embodiment of the present invention is not limited thereto. For example, a structure without the coloring film 836 may be employed in the case where the EL layer 846 is formed by separate coloring.

Next, a display device 700A that is a modification example of the display device 700 illustrated in FIG. 28 is described with reference to FIG. 30.

<Structure Example 2 of Display Device Using Liquid Crystal Element as Display Element>

The display device 700A in FIG. 30 includes the liquid crystal element 775. The liquid crystal element 775 includes a conductive film 773, a conductive film 777, and the liquid crystal layer 776. The conductive film 773 is provided over the planarization insulating film 770 over the first substrate 701 to function as a reflective electrode. The display device 700A in FIG. 30 is what is called a reflective color liquid crystal display device in which external light is reflected by the conductive film 773 to display an image through the coloring film 836.

Note that projections and depressions are provided in part of the planarization insulating film 770 of the pixel portion 702 in the display device 700A in FIG. 30. The projections and depressions can be formed in such a manner that the planarization insulating film 770 is formed using an organic resin film or the like, and projections and depressions are formed on the surface of the organic resin film. The conductive film 773 functioning as a reflective electrode is formed along the projections and depressions. Therefore, when external light is incident on the conductive film 773, the light is reflected diffusely at the surface of the conductive film 773, whereby visibility can be improved.

The display device 700A includes the light-blocking film 838, the insulating film 834, and the coloring film 836 on the second substrate 705 side. For the light-blocking film 838, the insulating film 834, and the coloring film 836, the materials and methods in the description of the display device 800 can be referred to. The conductive film 773 included in the display device 700A is electrically connected to the conductive films functioning as the source electrode or the drain electrode of the transistor 750. For the conductive film 773, the material and method in the description of the conductive film 844 can be referred to.

The display device 700A includes a capacitor 790. The capacitor 790 includes a pair of electrodes and an insulating film therebetween. Specifically, in the capacitor 790, a conductive film that is formed in the same process as a conductive film serving as a gate electrode, the source electrode, and the drain electrode of the transistor 750 is used as one electrode, a conductive film 792 that is formed in the same process as a conductive film serving as a lead wiring of the transistor 750 is used as the other electrode, and an insulating film 764 is provided between the conductive films.

In the display device 700A, a signal line 710 a, instead of the signal line 710 in the display device 700 illustrated in FIG. 28, is provided in the lead wiring portion 711. In addition, a connection electrode 760 a, instead of the connection electrode 760 in the display device 700 illustrated in FIG. 28, is provided in the FPC terminal portion 708 in the display device 700A. The signal line 710 a, the connection electrode 760 a, and the conductive film 792 are formed in the same process. For example, the signal line 710 a, the connection electrode 760 a, and the conductive film 792 can be formed in the same process by processing one conductive film.

In a transistor that is a semiconductor device of one embodiment of the present invention, a conductive film serving as a gate electrode and conductive films serving as a source electrode and a drain electrode are faulted in the same process. In other words, the conductive film serving as the gate electrode and the conductive films serving as the source electrode and the drain electrodes are formed on the same plane. For this reason, connection between the conductive films serving as the gate electrodes and/or connection between the conductive films serving as the source electrode and the drain electrode are/is established through another conductive film.

An example of a connection portion where the conductive film serving as the gate electrode is connected to the conductive films serving as the source electrode and the drain electrode is described with reference to FIGS. 37A and 37B.

FIG. 37A is a top view of a connection portion 900, and FIG. 37B is a cross-sectional view taken along the dashed-dotted line Z1-Z2 in FIG. 37A. Note that in FIG. 37B, some components such as an insulating film are not illustrated for clarity.

The connection portion 900 includes an insulating film 904 over a substrate 902, an insulating film 908 over the insulating film 904, conductive films 910, 912, and 914 over the insulating film 908, an insulating film 916 over the insulating film 908 and the conductive films 910, 912, and 914, a conductive film 945 connected to the conductive films 910 and 912 through openings 930 a and 930 b in the insulating film 916, and an insulating film 918 over the insulating film 916 and the conductive film 945.

The substrate 902 can be formed using a material similar to that of the substrate 102 described in Embodiment 1. The insulating films 904, 908, and 916 can be formed using materials similar to those of the insulating films 104, 108, and 116 described in Embodiment 1, respectively. The conductive films 910, 912, 914, and 945 can be formed using materials similar to those of the conductive films 110, 112, and 114 described in Embodiment 1.

The conductive film 910 is connected to a source electrode or a drain electrode of a transistor. The conductive film 912 is connected to a source electrode or a drain electrode of the transistor. The conductive film 945 is electrically connected to the conductive films 910 and 912 through the openings 930 a and 930 b.

Another example of a connection portion where the conductive film serving as the gate electrode is connected to the conductive films serving as the source electrode and the drain electrode is described with reference to FIGS. 40A and 40B.

FIG. 40A is a top view of the connection portion 900, and FIG. 40B is a cross-sectional view taken along the dashed-dotted line Z1-Z2 in FIG. 40A. Note that in FIG. 40B, some components such as an insulating film are omitted for clarity.

The conductive films 910 and 912 are connected to each other through a conductive film 120 a. The conductive film 120 a is formed and etched at the same time as the conductive film 120. Thus, the conductive films 120 and 120 a contain the same material.

As described above, the transistor that is the semiconductor device of one embodiment of the present invention can be used for a variety of display devices.

The structure described in this embodiment can be used in appropriate combination with any of the structures described in the other embodiments.

Embodiment 6

In this embodiment, a display device that can be formed using a semiconductor device of one embodiment of the present invention is described with reference to FIGS. 31A to 31C.

The display device illustrated in FIG. 31A includes a region including pixels of display elements (hereinafter the region is referred to as a pixel portion 502), a circuit portion being provided outside the pixel portion 502 and including a circuit for driving the pixels (hereinafter the portion is referred to as a driver circuit portion 504), circuits each having a function of protecting an element (hereinafter the circuits are referred to as protection circuits 506), and a terminal portion 507. Note that the protection circuits 506 are not necessarily provided.

A part or the whole of the driver circuit portion 504 is preferably formed over a substrate over which the pixel portion 502 is formed, in which case the number of components and the number of terminals can be reduced. When a part or the whole of the driver circuit portion 504 is not formed over the substrate over which the pixel portion 502 is formed, the part or the whole of the driver circuit portion 504 can be mounted by COG or tape automated bonding (TAB).

The pixel portion 502 includes a plurality of circuits for driving display elements arranged in X rows (X is a natural number of 2 or more) and Y columns (Y is a natural number of 2 or more) (hereinafter, such circuits are referred to as pixel circuits 501). The driver circuit portion 504 includes driver circuits such as a circuit for supplying a signal (scan signal) to select a pixel (hereinafter, the circuit is referred to as a gate driver 504 a) and a circuit for supplying a signal (data signal) to drive a display element in a pixel (hereinafter, the circuit is referred to as a source driver 504 b).

The gate driver 504 a includes a shift register or the like. The gate driver 504 a receives a signal for driving the shift register through the terminal portion 507 and outputs a signal. For example, the gate driver 504 a receives a start pulse signal, a clock signal, or the like and outputs a pulse signal. The gate driver 504 a has a function of controlling the potentials of wirings supplied with scan signals (hereinafter, such wirings are referred to as scan lines GL_1 to GL_X). Note that a plurality of gate drivers 504 a may be provided to control the scan lines GL_1 to GL_X separately. Alternatively, the gate driver 504 a has a function of supplying an initialization signal. Without being limited thereto, the gate driver 504 a can supply another signal.

The source driver 504 b includes a shift register or the like. The source driver 504 b receives a signal (video signal) from which a data signal is derived, as well as a signal for driving the shift register, through the terminal portion 507. The source driver 504 b has a function of generating a data signal to be written to the pixel circuit 501 which is based on the video signal. In addition, the source driver 504 b has a function of controlling output of a data signal in response to a pulse signal produced by input of a start pulse signal, a clock signal, or the like. Furthermore, the source driver 504 b has a function of controlling the potentials of wirings supplied with data signals (hereinafter such wirings are referred to as signal lines DL_1 to DL_Y). Alternatively, the source driver 504 b has a function of supplying an initialization signal. Without being limited thereto, the source driver 504 b can supply another signal.

The source driver 504 b includes a plurality of analog switches or the like, for example. The source driver 504 b can output, as the data signals, signals obtained by time-dividing the video signal by sequentially turning on the plurality of analog switches. The source driver 504 b may include a shift register or the like.

A pulse signal and a data signal are input to each of the plurality of pixel circuits 501 through one of the plurality of scan lines GL supplied with scan signals and one of the plurality of signal lines DL supplied with data signals, respectively. Writing and holding of the data signal to and in each of the plurality of pixel circuits 501 are controlled by the gate driver 504 a. For example, to the pixel circuit 501 in the m-th row and the n-th column (m is a natural number of less than or equal to X, and n is a natural number of less than or equal to Y), a pulse signal is input from the gate driver 504 a through the scan line GL_m, and a data signal is input from the source driver 504 b through the signal line DL_n in accordance with the potential of the scan line GL_m.

The protection circuit 506 shown in FIG. 31A is connected to, for example, the scan line GL between the gate driver 504 a and the pixel circuit 501. Alternatively, the protection circuit 506 is connected to the signal line DL between the source driver 504 b and the pixel circuit 501. Alternatively, the protection circuit 506 can be connected to a wiring between the gate driver 504 a and the terminal portion 507. Alternatively, the protection circuit 506 can be connected to a wiring between the source driver 504 b and the terminal portion 507. Note that the terminal portion 507 means a portion having terminals for inputting power, control signals, and video signals to the display device from external circuits.

The protection circuit 506 is a circuit that electrically connects a wiring connected to the protection circuit to another wiring when a potential out of a certain range is applied to the wiring connected to the protection circuit.

As illustrated in FIG. 31A, the protection circuits 506 are provided for the pixel portion 502 and the driver circuit portion 504, so that the resistance of the display device to overcurrent generated by electrostatic discharge (ESD) or the like can be improved. Note that the configuration of the protection circuits 506 is not limited to that, and for example, the protection circuit 506 may be configured to be connected to the gate driver 504 a or the protection circuit 506 may be configured to be connected to the source driver 504 b. Alternatively, the protection circuit 506 may be configured to be connected to the terminal portion 507.

In FIG. 31A, an example in which the driver circuit portion 504 includes the gate driver 504 a and the source driver 504 b is shown; however, the structure is not limited thereto. For example, only the gate driver 504 a may be formed and a separately prepared substrate where a source driver circuit is formed (e.g., a driver circuit substrate formed with a single crystal semiconductor film or a polycrystalline semiconductor film) may be mounted.

Each of the plurality of pixel circuits 501 in FIG. 31A can have the structure illustrated in FIG. 31B, for example.

The pixel circuit 501 illustrated in FIG. 31B includes a liquid crystal element 570, a transistor 550, and a capacitor 560.

The semiconductor device of one embodiment of the present invention can be used as, for example, the transistor 550. As the transistor 550, any of the transistors 150 to 154, the transistors 190 to 194, the transistor 150A, the transistor 190A, the transistor 190B, and the like described in the above embodiments can be used.

The potential of one of a pair of electrodes of the liquid crystal element 570 is set in accordance with the specifications of the pixel circuit 501 as appropriate. The alignment state of the liquid crystal element 570 depends on written data. A common potential may be supplied to one of the pair of electrodes of the liquid crystal element 570 included in each of the plurality of pixel circuits 501. Furthermore, the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in one row may be different from the potential supplied to one of the pair of electrodes of the liquid crystal element 570 in the pixel circuit 501 in another row.

As examples of a driving method of the display device including the liquid crystal element 570, any of the following modes can be given: a TN mode, an STN mode, a VA mode, an axially symmetric aligned micro-cell (ASM) mode, an optically compensated birefringence (OCB) mode, a ferroelectric liquid crystal (FLC) mode, an antiferroelectric liquid crystal (AFLC) mode, an MVA mode, a patterned vertical alignment (PVA) mode, an IPS mode, an FFS mode, a transverse bend alignment (TBA) mode, and the like. Other examples of the driving method of the display device include an electrically controlled birefringence (ECB) mode, a polymer dispersed liquid crystal (PDLC) mode, a polymer network liquid crystal (PNLC) mode, and a guest-host mode. Note that the present invention is not limited to these examples, and various liquid crystal elements and driving methods can be applied to the liquid crystal element and the driving method thereof.

In the pixel circuit 501 in the m-th row and the n-th column, one of a source electrode and a drain electrode of the transistor 550 is electrically connected to the signal line DL_n, and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. A gate electrode of the transistor 550 is electrically connected to the scan line GL_m. The transistor 550 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 560 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL), and the other is electrically connected to the other of the pair of electrodes of the liquid crystal element 570. The potential of the potential supply line VL is set in accordance with the specifications of the pixel circuit 501 as appropriate. The capacitor 560 functions as a storage capacitor for storing written data.

For example, in the display device including the pixel circuit 501 in FIG. 31B, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 31A, whereby the transistors 550 are turned on and a data signal is written.

When the transistors 550 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. This operation is sequentially performed row by row; thus, an image can be displayed.

Alternatively, each of the plurality of pixel circuits 501 in FIG. 31A can have, for example, the structure illustrated in FIG. 31C.

The pixel circuit 501 illustrated in FIG. 31C includes transistors 552 and 554, a capacitor 562, and a light-emitting element 572. Here, as one or both of the transistor 552 and the transistor 554, any of the transistors 150 to 154, the transistors 190 to 194, the transistor 150A, the transistor 190A, the transistor 190B, and the like described in the above embodiments can be used.

One of a source electrode and a drain electrode of the transistor 552 is electrically connected to a wiring to which a data signal is supplied (a signal line DL_n). A gate electrode of the transistor 552 is electrically connected to a wiring to which a gate signal is supplied (a scan line GL_m).

The transistor 552 has a function of controlling whether to write a data signal by being turned on or off.

One of a pair of electrodes of the capacitor 562 is electrically connected to a wiring to which a potential is supplied (hereinafter referred to as a potential supply line VL_a), and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

The capacitor 562 functions as a storage capacitor for storing written data.

One of a source electrode and a drain electrode of the transistor 554 is electrically connected to the potential supply line VL_a. Furthermore, a gate electrode of the transistor 554 is electrically connected to the other of the source electrode and the drain electrode of the transistor 552.

One of an anode and a cathode of the light-emitting element 572 is electrically connected to a potential supply line VL_b, and the other is electrically connected to the other of the source electrode and the drain electrode of the transistor 554.

As the light-emitting element 572, an organic electroluminescent element (also referred to as an organic EL element) or the like can be used, for example. Note that the light-emitting element 572 is not limited to an organic EL element; an inorganic EL element including an inorganic material may be used.

A high power supply potential VDD is supplied to one of the potential supply line VL_a and the potential supply line VL_b, and a low power supply potential VSS is supplied to the other.

For example, in the display device including the pixel circuit 501 in FIG. 31C, the pixel circuits 501 are sequentially selected row by row by the gate driver 504 a illustrated in FIG. 31A, whereby the transistors 552 are turned on and a data signal is written.

When the transistors 552 are turned off, the pixel circuits 501 in which the data has been written are brought into a holding state. Furthermore, the amount of current flowing between the source electrode and the drain electrode of the transistor 554 is controlled in accordance with the potential of the written data signal. The light-emitting element 572 emits light with a luminance corresponding to the amount of flowing current. This operation is sequentially performed row by row; thus, an image can be displayed.

The structure described in this embodiment can be combined as appropriate with any of the structures described in the other embodiments.

Embodiment 7

In this embodiment, a display module and electronic appliances that can be formed using a semiconductor device of one embodiment of the present invention are described with reference to FIG. 32 and FIGS. 33A to 33H.

In a display module 8000 illustrated in FIG. 32, a touch panel 8004 connected to an FPC 8003, a display panel 8006 connected to an FPC 8005, a backlight 8007, a frame 8009, a printed board 8010, and a battery 8011 are provided between an upper cover 8001 and a lower cover 8002.

The semiconductor device of one embodiment of the present invention can be used for, for example, the display panel 8006.

The shapes and sizes of the upper cover 8001 and the lower cover 8002 can be changed as appropriate in accordance with the sizes of the touch panel 8004 and the display panel 8006.

The touch panel 8004 can be a resistive touch panel or a capacitive touch panel and can be formed to overlap the display panel 8006. A counter substrate (sealing substrate) of the display panel 8006 can have a touch panel function. A photosensor may be provided in each pixel of the display panel 8006 to form an optical touch panel.

The backlight 8007 includes a light source 8008. Note that although a structure in which the light sources 8008 are provided over the backlight 8007 is illustrated in FIG. 32, one embodiment of the present invention is not limited to this structure. For example, a structure in which the light source 8008 is provided at an end portion of the backlight 8007 and a light diffusion plate is further provided may be employed. Note that the backlight 8007 need not be provided in the case where a self-luminous light-emitting element such as an organic EL element is used or in the case where a reflective panel or the like is employed.

The frame 8009 protects the display panel 8006 and also functions as an electromagnetic shield for blocking electromagnetic waves generated by the operation of the printed board 8010. The frame 8009 may function as a radiator plate.

The printed board 8010 is provided with a power supply circuit and a signal processing circuit for outputting a video signal and a clock signal. As a power source for supplying power to the power supply circuit, an external commercial power source or a power source using the battery 8011 provided separately may be used. The battery 8011 can be omitted in the case of using a commercial power source.

The display module 8000 may be additionally provided with a member such as a polarizing plate, a retardation plate, or a prism sheet.

FIGS. 33A to 33H illustrate electronic appliances. These electronic appliances can include a housing 5000, a display portion 5001, a speaker 5003, an LED lamp 5004, operation keys 5005 (including a power switch or an operation switch), a connection terminal 5006, a sensor 5007 (a sensor having a function of measuring or sensing force, displacement, position, speed, acceleration, angular velocity, rotational frequency, distance, light, liquid, magnetism, temperature, chemical substance, sound, time, hardness, electric field, current, voltage, electric power, radiation, flow rate, humidity, gradient, oscillation, odor, or infrared ray), a microphone 5008, and the like.

FIG. 33A illustrates a mobile computer that can include a switch 5009, an infrared port 5010, and the like in addition to the above components. FIG. 33B illustrates a portable image reproducing device (e.g., a DVD player) that is provided with a memory medium and can include a second display portion 5002, a memory medium reading portion 5011, and the like in addition to the above components. FIG. 33C illustrates a goggle-type display that can include the second display portion 5002, a support 5012, an earphone 5013, and the like in addition to the above components. FIG. 33D illustrates a portable game machine that can include the memory medium reading portion 5011 and the like in addition to the above components. FIG. 33E illustrates a digital camera that has a television reception function and can include an antenna 5014, a shutter button 5015, an image receiving portion 5016, and the like in addition to the above components. FIG. 33F illustrates a portable game machine that can include the second display portion 5002, the memory medium reading portion 5011, and the like in addition to the above components. FIG. 33G illustrates a television receiver that can include a tuner, an image processing portion, and the like in addition to the above components. FIG. 33H illustrates a portable television receiver that can include a charger 5017 capable of transmitting and receiving signals, and the like in addition to the above components.

The electronic appliances illustrated in FIGS. 33A to 33H can have a variety of functions, for example, a function of displaying a variety of data (a still image, a moving image, a text image, and the like) on the display portion, a touch panel function, a function of displaying a calendar, date, time, and the like, a function of controlling a process with a variety of software (programs), a wireless communication function, a function of being connected to a variety of computer networks with a wireless communication function, a function of transmitting and receiving a variety of data with a wireless communication function, a function of reading a program or data stored in a memory medium and displaying the program or data on the display portion, and the like. Furthermore, the electronic appliance including a plurality of display portions can have a function of displaying image data mainly on one display portion while displaying text data on another display portion, a function of displaying a three-dimensional image by displaying images on a plurality of display portions with a parallax taken into account, or the like. Furthermore, the electronic appliance including an image receiving portion can have a function of shooting a still image, a function of taking a moving image, a function of automatically or manually correcting a shot image, a function of storing a shot image in a memory medium (an external memory medium or a memory medium incorporated in the camera), a function of displaying a shot image on the display portion, or the like. Note that functions that can be provided for the electronic appliances illustrated in FIGS. 33A to 33H are not limited to those described above, and the electronic appliances can have a variety of functions.

The electronic appliances described in this embodiment each include the display portion for displaying some sort of data. Note that the semiconductor device of one embodiment of the present invention can also be used for an electronic appliance that does not have a display portion.

The structure described in this embodiment can be combination as appropriate with any of the structures described in the other embodiments.

This application is based on Japanese Patent Application serial no. 2013-271312 filed with the Japan Patent Office on Dec. 27, 2013 and Japanese Patent Application serial no. 2014-037207 filed with the Japan Patent Office on Feb. 27, 2014, the entire contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A semiconductor device comprising: an oxide insulating film; an oxide semiconductor film over the oxide insulating film, the oxide semiconductor film comprising a first region and a second region; a source electrode and a drain electrode in contact with the oxide semiconductor film; a gate insulating film between the source electrode and the drain electrode; a gate electrode overlapping the oxide semiconductor film with the gate insulating film therebetween; and a nitride insulating film overlapping the first region with the gate insulating film therebetween, wherein the nitride insulating film is on and in contact with the gate electrode, the source electrode and the drain electrode, wherein the first region and the gate electrode overlap each other, wherein the nitride insulating film is in contact with the second region, wherein a concentration of an impurity element of the first region and a concentration of the impurity element of the second region are different, and wherein the gate electrode, the source electrode, and the drain electrode comprise the same metal element.
 2. The semiconductor device according to claim 1, wherein the first region comprises a first oxide semiconductor layer and a second oxide semiconductor layer overlapping each other.
 3. The semiconductor device according to claim 2, wherein the oxide semiconductor film further comprises a third oxide semiconductor layer overlapping with the first oxide semiconductor layer and the second oxide semiconductor layer.
 4. The semiconductor device according to claim 1, wherein the concentration of the impurity element of the second region is higher than the concentration of the impurity element of the first region.
 5. The semiconductor device according to claim 1, wherein the impurity element is selected from the group consisting of hydrogen, boron, carbon, nitrogen, fluorine, aluminum, silicon, phosphorus, and chlorine.
 6. The semiconductor device according to claim 1, wherein the oxide semiconductor film comprises an oxide comprising In, M, and Zn, and wherein M is selected from the group consisting of Ti, Ga, Y, Zr, La, Ce, Nd, and Hf.
 7. The semiconductor device according to claim 1, wherein the nitride insulating film is a silicon nitride film.
 8. The semiconductor device according to claim 1, wherein at least a part of the gate electrode and parts of the source electrode and the drain electrode exist in the same layer.
 9. The semiconductor device according to claim 1, wherein the nitride insulating film covers and is in contact with the gate electrode, the source electrode, and the drain electrode.
 10. A display device comprising the semiconductor device according to claim
 1. 